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At least 379 records · Page 21Linked to original sources
USGS1999

High-grade iron ore deposits of the Mesabi Range, Minnesota-product of a continental-scale proterozoic ground-water flow system

The Mesabi Range along the north edge of the Paleoproterozoic Penokean orogen in northern Minnesota has produced 3.6 billion metric tons of ore since its discovery in 1890. Of that amount, 2.3 billion metric tons were extracted from hematite-or goethite-rich deposits generally referred to as "high-grade" ores. The high-grade ores formed as the Biwabik Iron-Formation was oxidized, hydrated, and leached by solutions flowing along open faults and fractures. The source of the ore-forming solutions has been debated since it was first proposed that the ores were weathering products formed by descending meteoritic ground-water flowing in late Mesozoic time. Subsequently others believed that the ores were better explained by ascending solutions, possibly hydrothermal solutions of pre-Phanerzoic age. Neither Wolff nor Gruner could reconcile their observations with a reasonable source for the solutions. In this paper, I build on modern mapping of the Mesabi Range and mine-specific geologic observations summarized in the literature to propose a conceptual model in which the high-grade ores formed from ascending solutions that were part of a continent-scale topographic or gravity-driven ground-water system. I propose that the ground-water system was active during the later stages of the development of a coupled fold and thrust belt and foreland basin that formed during the Penokean orogen.

Economic Geology↗
USGS2008

Tertiary tilting and dismemberment of the laramide arc and related hydrothermal systems, Sierrita Mountain, Arizona

Multiple lines of evidence, including new and published geologic mapping and paleomagnetic and geobarometric determinations, demonstrate that the rocks and large porphyry copper systems of the Sierrita Mountains in southern Arizona were dismembered and tilted 50?? to 60?? to the south by Tertiary normal faulting. Repetition of geologic features and geobarometry indicate that the area is segmented into at least three major structural blocks, and the present surface corresponds to oblique sections through the Laramide plutonic-hydrothermal complex, ranging in paleodepth from ???1 to ???12 km. These results add to an evolving view of a north-south extensional domain at high angles to much extension in the southern Basin and Range, contrast with earlier interpretations that the Laramide systems are largely upright and dismembered by thrust faults, highlight the necessity of restoring Tertiary rotations before interpreting Laramide structural and hydrothermal features, and add to the broader understanding of pluton emplacement and evolution of porphyry copper systems. ?? 2008 Society of Economic Geologists, Inc.

Economic Geology↗
USGS1948

Geology and ground water of the Casablanca Basin, Chile

"In the uplands the Cretaceous granodiorite and granite contain water in the weathered zones, and in joints and fractures. These yield small supplies of good water to springs and to shallow domestic and stock wells. The alluvial fill of the lowlands contains a zone of saturation that is sustained by infiltration from rainfall and from the runoff of the uplands.... Where present in the zone of saturation, the coarse-grained members of the alluvial fill yield moderate to large supplies of good water to wells."

Casablanca Basin↗
USGS2019

The Mississippi Alluvial Plain aquifers—An engine for economic activity

U.S. Geological Survey science supports groundwater resource management in the Mississippi Alluvial Plain region. The USGS Science and Decisions Center is working with the Water Availability and Use Science Program to integrate economics into a sophisticated model of groundwater in the region. The model will quantify the status of the groundwater system and help researchers, stakeholders, and decision-makers understand and manage groundwater resources. Including economics in the model will let users consider the influence of groundwater levels on regional economics and the effects of economic factors on the demand for groundwater.

Mississippi Alluvial Plain↗
USGS1950

Geochemistry of ground water

The science of geochemistry is the study of the chemical composition of and actual or possible chemical changes in the crust of the earth. The subdivision of geochemistry concerned with ground water includes study of the nature and amounts of dissolved mineral matter in ground waters, the chemical and geologic changes that influence the kinds and amounts of substances carried in solution, and the chemical relationships between dissolved mineral matter in waters and minerals that make up the rocks in which the waters occur. As . thus defined, the field of geochemistry of ground water is very large. Although much work has been done in geochemistry in general, and in some phases of the geochemistry of ground water, the field remains to a large extent unexplored.

Economic Geology↗
USGS1950

Effects of earthquakes, trains tides, winds, and atmospheric pressure changes on water in the geologic formations of southern Florida

Determination of such fundamental hydrologic factors as the coefficients of permeability, transmissibility, and storage; areas of recharge and discharge; direction of ground-water movement; the safe yield; and other pertinent, related factors are based in part upon water-level measurements in wells. But these water-level readings, if not properly understood or weighted, may be entirely misleading. This is because water levels are neither stationary nor steadily rising (with recharge) or falling (with discharge). Water levels fluctuate as a result of several factors other than those of changes in recharge or discharge, chief among which are: (1) tides; (2) atmospheric pressure; (3) winds; (4) earthquakes; and (5) passing trains. Characteristically, artesian aquifers respond to these forces in a manner different from that of nonartesian aquifers, but in some places local geologic conditions in a nonartesian aquifer may cause water-level fluctuations ordinarily found only in artesian wells. Among the unusual features observed is a 4.5-foot fluctuation of water level in a Miami well caused by earthquake waves originating 750 miles away in the Atlantic Ocean, off the coast of the Dominican Republic. Quakes from all over the world are responsible for water-level fluctuations in this and other wells in the Miami area.

Florida↗
USGS2003

Gold deposits in metamorphic belts: Overview of current understanding, outstanding problems, future research, and exploration significance

Metamorphic belts are complex regions where accretion or collision has added to, or thickened, continental crust. Gold-rich deposits can be formed at all stages of orogen evolution, so that evolving metamorphic belts contain diverse gold deposit types that may be juxtaposed or overprint each other. This partly explains the high level of controversy on the origin of some deposit types, particularly those formed or overprinted/remobilized during the major compressional orogeny that shaped the final geometry of the hosting metamorphic belts. These include gold-dominated orogenic and intrusion-related deposits, but also particularly controversial gold deposits with atypical metal associations. There are a number of outstanding problems for all types of gold deposits in metamorphc belts. These include the following: (1) definitive classifications, (2) unequivocal recognition of fluid and metal sources, (3) understanding of fluid migration and focusing at all scales, (4) resolution of the precise role of granitoid magmatism, (5) precise gold-depositional mechanisms, particularly those producing high gold grades, and (6) understanding of the release of CO2-rich fluids from subducting slabs and subcreted oceanic crust and granitoid magmas at different crustal levels. Research needs to be better coordinated and more integrated, such that detailed fluid-inclusion, trace-element, and isotopic studies of both gold deposits and potential source rocks, using cutting-edge technology, are embedded in a firm geological framework at terrane to deposit scales. Ultimately, four-dimensional models need to be developed, involving high-quality, three-dimensional geological data combined with integrated chemical and fluid-flow modeling, to understand the total history of the hydrothermal systems involved. Such research, particularly that which can predict superior targets visible in data sets available to exploration companies before discovery, has obvious spin-offs for global- to deposit-scale targeting of deposits with superior size and grade in the covered terranes that will be the exploration focus of the twenty-first century.

Economic Geology↗
USGS1949

Geochemical prospecting for ores; a progress report

Methods of prospecting for mineral deposits by means of chemical studies of residual soil, alluvium, glacial moraine, vegetation, and natural water were first applied on a systematic scale in Scandinavia and Russia about 15 years ago. Since the war, work in this field has been undertaken by several independent groups in the United States and Canada, including the Geochemical Prospecting Unit of the U. S. Geological Survey.Best results have been obtained by systematic analysis of zinc in residuum, ground water, and vegetation; copper and lead in gossan and residuum; tin in residuum and alluvium; molybdenum in residuum; and cobalt, nickel, and gold in vegetation. In addition, studies of plant ecologies and toxicity symptoms in vegetation in relation to metal-rich soil appear to be promising.Rapid colorimetric and spectrographic tests for traces of metals in soil, water, and vegetation have been developed for use under field conditions.

Economic Geology↗
USGS1969

Experimental studies of pegmatite genesis: I. A model for the derivation and crystallization of granitic pegmatites

The genesis of granitic igneous pegmatites is here considered in terms of a model conceived from results of field and laboratory studies and subsequently tested by means of experimental investigations. This model emphasizes the roles of water (and/or other relatively volatile substances), both as a dissolved constituent in granitic magmas and as the dominant constituent of a separate fluid phase that is in the supercritical state under most conditions of pegmatite formation. Pegmatite magma, as distinguished by a content of dissolved water that is high relative to the limit of solubility under existing confining pressure, can be formed either through partial melting of crustal materials or as rest-liquid in a cooling igneous body yielding dominantly anhydrous crystalline phases. Such granitic magma can be expected to consolidate according to the following three-fold sequence: 1. Crystallization from hydrous silicate melt, yielding anhydrous solid phases with or without OH-bearing phases. The product is characterized by normal phaneritic textures that generafly are coarse grained. It has been termed pegmatite in some occurrences, and granite in others. 2. Crystallization concomitantly from silicate melt and from a coexisting exsolved aqueous fluid of considerably lower viscosity, yielding giant-textured pegmatite along with much finer-grained, even aplitic, mineral aggregates. Segregation of these products can vary enormously in scale and degree. Partitioning of constituents between melt and aqueous fluid, rapid diffusion of constituents through the aqueous phase, and gravitational rising of this fluid through the system contribute to formation of pods, zones, and other rock units of unusual composition and texture. 3. Crystallization in the absence of silicate melt, yielding a wide variety of late-stage products. These include so-called "pocket minerals" and numerous mineral aggregates formed through exchanges of material among aqueous fluid and earlier-formed crystal-line phases. Development of pegmatite bodies can begin with either Step 1 or Step 2, but it is suggested that the processes involved in Step 2 are essential to the formation of all true pegmatites of igneous origin. The appearance of a second fluid phase, in general a supercritical aqueous fluid derived from the crystallizing melt, is regarded as the decisive event; it is promptly followed by fundamental changes in distribution and texture of the solid phases being formed. The processes can operate effectively in a fully closed system, and they also can modify the surrounding rocks if the system is open at any stage. Step 1 can include reactions between magma and earlier-formed crystals, but far more rapid and extensive exchanges of materials are subsequently effected by processes included in Steps 2 and 3; indeed, such exchanges also can account satisfactorily for pegmatites of metamorphic origin. Crystallization of most granitic magmas in the absence of a separate aqueous phase probably would begin within the temperature range 1,300°-650 ° C, the specific liquidus temperature depending mainly upon the amounts of volatile constituents held in solution at the time. This compositional factor also would be important in controlling the stage of crystallization-late, intermediate, or early-at which a separate aqueous fluid would make its appearance. Depending upon confining pressure as dictated by geologic conditions for a given system, the stage in crystallization represented by the presence of both silicate melt and aqueous fluid could begin within about the same temperature range of 1.300°-650 ° C. Exhaustion of the melt could occur within range extending downward to temperatures of 600C or even somewhat lower. Textural and structural features appear to be the most reliable indicators of the stages and fundamental processes involved in crystallization of both natural and synthetic pegmatites. The contrasting processes of crystallization from one fluid and from more than one fluid can operate over such broad P-T-X ranges that simple genetic pegmatite classifications based largely upon "key minerals," presumed temperature or pressure intervals, or the presence or absence of supercritical conditions appear to be somewhat unrealistic. © 1969 Society of Economic Geologists, Inc.

Economic Geology↗
USGS1978

Remote-sensing and subsurface definition of facies and structure related to uranium deposits, Powder River Basin, Wyoming

Computer-enhanced Landsat images of the southern Powder River Basin have been used to define facies and linear structural features within the Wasatch Formation. The facies distribution is detectable primarily because of a relation of vegetation density and type to the local substrate. The surface indications of facies are confirmed by sandstone/ mudstone ratios determined from logs of abundant exploration drill holes. These newly defined geologic features are spatially related to known uranium mineral occurrences and are believed to be related to mineralization in the following ways. (1) Major uranium occurrences are virtually restricted to an intermediate grain-size facies of the Wasatch, probably marking the axial zone of the depositional basin. (2) The axial zone is also marked by a change from one structural lineament domain to another, and the structures may have influenced details of fluvial-system patterns and sedimentation and (subsequently) the flow of uranium-bearing ground water. (3) A recently active linear structure may mark the current basin axis; it appears to have some relationship both to groundwater chemistry and the distribution of uranium occurrences, suggesting structural influence on relatively modern ground-water transport and uranium deposition. © 1978 Society of Economic Geologists, Inc.

Wyoming↗
USGS2002

Age and origin of base and precious metal veins of the Coeur d'Alene mining district, Idaho

Ore-bearing quartz-carbonate veins of the Coeur d'Alene mining district yield 87Sr/86Sr ratios of 0.74 to >1.60 for low Rb/Sr, carbonate gangue minerals, similar to current ranges measured in Middle Proterozoic, high Rb/Sr rocks of the Belt Supergroup. Stable isotope and fluid inclusion studies establish a genetic relationship between vein formation and metamorphic-hydrothermal systems of the region. These extraordinary 87Sr/86Sr ratios require accumulation of radiogenic 87Sr in a high Rb/Sr system over an extended period prior to incorporation of Sr into the hydrothermal veins. Evaluation of the age and composition of potential sources of highly radiogenic Sr indicates that the ore-bearing veins of the Coeur d'Alene district formed during the Cretaceous from components scavenged from rocks of the Belt Supergroup, the primary host rocks of the district. Proterozoic Pb isotope ratios observed in galena from many Coeur d'Alene veins were established when Pb separated from uranium during deposition or diagenesis of the Belt Supergroup at 1400 to 1500 Ma, possibly as disseminated syngenetic deposits. K-Ar and Rb-Sr apparent ages and ??18O values of Belt Supergroup rocks decrease from the Coeur d'Alene district toward the Idaho and Kaniksu batholiths, approximately normal to the trends of metamorphic isograds, fold axes, foliation, and the major reverse faults of the district. Isoclinal folding, thrust faulting, high-temperature metamorphism, granitic plutonism, and regional-scale metamorphic-hydrothermal activity is documented in the region between 140 and 45 Ma, representing the only such combination of events in the Coeur d'Alene region subsequent to about 1300 Ma. The Sr and oxygen results and geologic evidence favor formation of the ore-bearing carbonate veins by fluids related to a complex metamorphic-hydrothermal system during the Cretaceous. Pb with Proterozoic isotopic compositions was probably mobilized and incorporated like other metals into the hydrothermal veins during this event. The ore-bearing veins were sheared and displaced during early Tertiary northwest-trending dextral strike-slip faulting along the Osburn fault and related structures of the Lewis and Clark line.

Economic Geology↗
USGS2019

Geochemistry and geophysics of iron oxide-apatite deposits and associated waste piles with implications for potential rare earth element resources from ore and historic mine waste in the eastern Adirondack Highlands, New York, USA

The iron oxide-apatite (IOA) deposits of the eastern Adirondack Highlands, New York, are historical high-grade magnetite mines that contain variable concentrations of rare earth element (REE)-bearing apatite crystals. The majority of the deposits are hosted within sodically altered Lyon Mountain granite gneiss, although some deposits occur within paragneiss, gabbro, anorthosite, or potassically altered Lyon Mountain granite gneiss. The IOA deposits and the waste and/or tailings piles associated with them have potential as an unconventional resource for REEs. Reprocessing of these piles would have the advantage of partial recycling of the waste material to produce a set of critical elements. Thirty-four ore, nine rock, 25 waste-pile, and four tailings-pile samples were collected and analyzed for major, minor, and trace elements. At the tailings- and waste-pile sites, composite samples were collected by combining 30 to >50 subsamples randomly distributed over each pile. The total REE content of the waste and tailings piles varied from approximately 10 to 22,000 ppm, whereas the ore sample concentrations ranged from approximately 15 to 48,000 ppm total REEs. A positive correlation exists between the total REE content of ore and its associated waste pile. Median light REE/heavy REE values were 2.14 for waste/tailings piles and 2.25 for ore, which is a substantial relative enrichment in the heavy REEs in comparison to many developed REE mines, such as the mined carbonatites of Bayan Obo, China, and Mountain Pass, California. Importantly, the ore and waste samples are significantly enriched in both Y and Nd compared to other REEs in the samples. Other minor components such as Th are also elevated. Airborne radiometric surveys show large positive eTh and eU anomalies corresponding to tailings piles. Although it is a limited data set, geochemical data of unaltered and altered host rocks suggest a speculative new model for IOA ore formation in the Adirondack Highlands that is consistent with the geology and previously published data. The ferroan ore-hosting Lyon Mountain granite gneiss underwent localized potassic alteration that enriched the altered rock in Fe, REEs, Th, and other metals. A later sodic alteration event affected the previously potassically altered Lyon Mountain granite gneiss, which increased rock porosity and remobilized Fe, REEs, and other elements from the host rock into the iron ore seams. The sodic fluids responsible for ore formation were enriched in F and Cl.

New York↗
USGS1971

Tertiary limestone aquifer system in the southeastern states

The hydrogeologic history of the Tertiary limestone system of the Southeastern States is reconstructed, especially as it relates to circulation of ground water and the development of solution cavities. The development of these solution cavities resembles in many respects the development of cavities in carbonates of the Knox Group of Tennessee during Middle Ordovician time, the cavities in the Knox having since been filled with collapse breccia that has been recemented. Some general principles of the circulation of water in limestone terranes and the related development of solution openings are reviewed so that a generic basis for comparison can be made of the modern southeast carbonate setting with the Ordovician carbonate setting in Tennessee.The major requirements for solutional development as cavities--(1) presence of highly soluble material, (2) a fracture system or some other form of incipient permeability, (3) water undersaturated with respect to soluble rocks, such as recharge from precipitation, and (4) hydraulic gradient--are found in the Tertiary limestone terrane of the southeast; much of the limestone has been elevated above sea level as a homoclinal seaward-dipping unit. Such a broad homoclinal setting, which also existed in the Knox at the close of early Ordovician time, facilitates extensive solutional development in the upper part of the zone of saturation. Circulation of water great enough to form a significant cavern network requires concentrated discharge areas, commonly as entrenched permanent streams or near-shore springs and seepage. This condition prevails where the Tertiary limestone is fairly close to land surface.Reconstruction of the geologic and hydrogeologic history of a carbonate region generally reveals the extent of early solution and karst development in relation to current karstification. Caverns, partly filled in some cases with loose or poorly cemented rock fragments that have fallen from cavern roofs, give evidence of karstification that is either current or that probably developed since the last marine inundation of the carbonate terrane. On the other hand, filling of caverns with overlying rock debris and reconstituting the debris into breccia are conditions that require evaluation of paleohydrology.

Alabama, Georgia, Florida, North Carolina, South C↗
USGS1984

Banded iron-formations of late Proterozoic age in the central eastern desert, Egypt: Geology and tectonic setting

In the central Eastern Desert of Egypt, deposits of iron-formation of the Algoma type occur as sharply defined stratigraphic units within layered volcanogenic rocks of late Proterozoic age. The volcanic sequence is characterized by interfingering and repetition of rocks of dominantly andesite-basalt composition and by tectonically juxtaposed ophiolitelike assemblages; it is intruded by voluminous syntectonic granodiorite-tonalite and post-tectonic potassic granite. The iron deposits were formed as chemical precipitates during lulls in dominantly subaqueous, calc-alkaline volcanism, apparently in an intraoceanic island-arc environment. During subsequent complex telescoping of the sequence, the rocks were transported along thrust faults and are now allochthonous.

Economic Geology↗
USGS2022

Early Neoproterozoic gold deposits of the Alto Guaporé province, southwestern Amazon craton, western Brazil

The Alto Guaporé gold province, southwestern Amazon craton, contains gold deposits that have been mined since the beginning of the 18th century and these deposits, together, have modern-day, pre-mining gold resources of at least 1.8 Moz. The ore is associated with quartz vein systems along the southeastern part of the Aguapei belt, a ~35-km-wide and ~500-km-long, NNW-trending shear zone formed due to reactivation of a terrane-bounding suture. The Aguapei belt evolved by ca. 1150 to 1100 Ma rifting and deposition of siliciclastic sediments in an aulacogen basin, followed by deformation and low-grade metamorphism of the sedimentary sequences during 1100 to 900 Ma terrane collision along the craton margin. The deformation was characterized by a compressional regime until ca. 950 Ma and transition to a transpressional setting during the final 50 m.y. The gold deposits are hosted in a variety of structures that are second-order to the main Aguapei shear zone. The Ernesto and Pau-a-Pique deposits are located ~40 km apart and at jogs along the Aguapei belt. They are marginal to pre-ore igneous rocks, with Ernesto hosted in the basal part of the metasedimentary Fortuna Formation that overlies tonalite and Pau-a-Pique at the contact between metasedimentary rocks and diorite. Three deformational phases comprise the compressional (D 1 to D 2 ) to transpressional (D 3 ) tectonic events. In the Pau-a-Pique deposit and the deeper level of the Ernesto deposit, the ore-bearing veins are bedding parallel and follow D 2 strike-slip and reverse fault zones, respectively. However, the veins formed during D 3 reactivation of the older structures by an array of oblique accommodation faults. In contrast, ores at shallower levels of Ernesto, both in discordant and bedding-parallel veins, are hosted within a ~20-m-thick rigid metaconglomerate with associated dilation due to the structural complexity as sedimentary rocks of the Aguapei Group were folded around the dome-shaped roof of the pre-ore tonalite. The ores in both deposits, as well as in many other deposits of the province, are characterized by disseminated and vein-hosted pyrite. Gold occurs mainly as inclusions in the pyrite, with other hydrothermal phases comprising muscovite, Fe-Ti oxides, and minor apatite, chalcopyrite, and galena. Fluid inclusion data, coupled with stable isotope geochemistry and geothermometry, indicate that gold precipitated from a low-salinity, CO 2 -rich fluid at ~300°C and ~2.5 kbar. The source for the fluid and gold was the interbedded pelites during devolatilization of the Aguapei Group sequence. The aqueous-carbonic fluid inclusions and the narrow range of δ 18 O values of quartz (12 ± 1 ‰ ) from many auriferous veins from the central part of the province represent a regional ore-forming fluid. The broad range of δ D for hydrous minerals (–116 to –55 ‰ ) reflects influx of small amounts of meteoric water into the steeply dipping shear zones during postgold exhumation. The 40 Ar/ 39 Ar geochronology from hydrothermal muscovite indicates a widespread hydrothermal event along the belt between 928 and 920 Ma. Collectively, the geological, geochronological, and geochemical data suggest that metamorphic fluids migrated laterally into and then upward along the Aguapei belt and deposited gold in lower-order structures where strain gradients existed between lithounits. The province has many characteristics of large orogenic gold provinces worldwide and represents a highly prospective and underexplored target region for early Neoproterozoic gold, a time period that generally is not well endowed in gold ores.

Alto Guaporé gold province, Amazon craton↗
USGS1998

Evidence for Proterozoic and late Cretaceous-early Tertiary ore-forming events in the Coeur d'Alene district, Idaho and Montana

New 40 Ar/ 39 Ar age spectra on sericite and lead isotope data on tetrahedrite, siderite, galena, bournonite, and stibnite, together with previously published isotopic, geochemical, and geologic studies provide evidence for two major vein-forming events in the Coeur d'Alene district and surrounding areas of the Belt basin. The data suggest that the zinc- and lead-rich veins (e.g., Bunker Hill and Star-Morning mines) formed in the Proterozoic (1.0 Ga), whereas the silver-rich veins (e.g., Silver belt mines), antimony veins (e.g., U.S. Antimony mine), and gold-bearing quartz veins (Murry subdistrict) formed in Late Cretaceous to early Tetitary time.

Economic Geology↗
USGS1947

The beryl resources of Connecticut

In 1942-44, about 120 Connecticut pegmatites were examined for beryl and other minerals during investigations by the Federal Geological Survey. Most of the pegmatites lie in the Middletown district, occurring principally in the Bolton schist and Monson gneiss. The pegmatites range from distinctly zoned bodies to those that are essentially uniform mixtures of the component minerals. Beryl in the zoned pegmatites commonly is present in greater concentration in one or more zones than in others. It occurs in the majority of pegmatites examined, but on the basis of crystal counts only 10 pegmatites, either in whole or in sizable part, average 0.2% or more beryl. Six structural types of beryl deposits are described and illustrated: border-zone, wall-zone, intermediate-zone, pod, fracture-controlled, and disseminated. Some pegmatites contain two or more types.Grades and tonnages of 14 beryl-bearing pegmatites are tabulated. Total indicated and inferred beryl-bearing pegmatite is 207,885 tons containing 859 + tons of beryl. Most of the beryl is in crystals too small to be economically separable by hand sorting, and barring a marked increase in the price of beryl, milling operations would have to be supported primarily by the yield of salable feldspar, scrap mica, and possibly quartz.

Connecticut↗
USGS1937

Recent geologic studies on Long Island with respect to ground-water supplies

Recent studies have shown that relatively impermeable clay beds are widespread on Long Island but that erosion channels cutting through them permit restricted recharge of the underlying beds in some parts of the island. Of the more than 200,000,000 gallons of water a day now pumped from wells, about 65 per cent. comes from the surficial beds of Illinoian or Wisconsin age. Because of the restricted recharge of the lower beds and the desirability of saving these beds for use in localities where the upper beds may more easily be contaminated by salt water, any future large developments of ground water should be drawn from the surficial beds.

New York↗