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A.H. Chidester

Publications and source records attributed to A.H. Chidester.

16 recordsLinked to original sources
USGS1974

Geology of the Harper Quadrangle, Liberia

As part of a program undertaken cooperatively by the Liberian Geological Survey (LGS) and the U. S. Geological Survey (USGS), under the sponsorship of the Government of Liberia and the Agency for International Development, U. S. Department of State, Liberia was mapped by geologic and geophysical methods during the period 1965 to 1972. The resulting geologic and geophysical maps are published in ten folios, each covering one quadrangle (see index map). The first systematic mapping in the Harper quadrangle was by Baker, S. P. Srivastava, and W. E. Stewart (LGS) at a scale of 1:500,000 in the vicinity of Harper in the southeastern, and of Karloke in the northeastern part of the quadrangle in 1960-61. Brock and Chidester carried out systematic mapping of the quadrangle at a scale of 1:250,000 in the period September 1971-May 1972; the geologic map was compiled from field data gathered by project geologists and private companies as indicated in the source diagram, photogeologic maps, interpretation of airborne magnetic and radiometric surveys, field mapping, and ground-based radiometric surveys in which hand-held scintillators were used. R. W. Bromery, C. S. Wotorson, and J. C. Behrendt contributed to the interpretation of geophysical data. Total-intensity aeromagnetic and total-count gamma radiation maps (Behrendt and Wotorson, in press a, b), and unpublished data derived from those maps, including the near-surface and the regional magnetic components and aeromagnetic/radiometric correlations, were used in the interpretation.

Harper Quadrangle↗
USGS1963

Bedrock Geology and Asbestos Deposits of the Upper Missisquoi Valley and Vicinity, Vermont

The upper Missisquoi Valley and vicinity as described in this report covers an area of about 250 square miles at the headwaters of the Missisquoi River in north-central Vermont. About 90 percent of the area is forested and the remainder is chiefly farm land. The topography reflects the geologic structure and varied resistance of the bedrock to erosion. Most of the area is on the east limb of the Green Mountain anticlinorium, which is the principal structural feature of Vermont. The bedrock is predominantly sedimentary and volcanic rock that has been regionally metamorphosed. It was intruded before metamorphism by mafic and ultramafic igneous rocks, and after metamorphism by felsic and mafic igneous rocks. The metamorphosed sedimentary and volcanic rocks range in age from Cambrian(?) to Middle Silurian, the intrusive igneous rocks from probably Late Ordovician to probably late Permian. Metamorphism and principal folding in the region occurred in Middle Devonian time. The metamorphosed sedimentary and volcanic rocks make up a section at least 25,000 feet thick and can be divided into nine formations. The Hazens Notch formation of Cambrian(?) and Early Cambrian age is characterized by carbonaceous schist. It is succeeded in western parts of the area by the Jay Peak formation of Early Cambrian age, which is chiefly a schist that is distinguished by the general absence of carbonaceous zones; in central parts of the area the Hazens Notch formation is followed by the Belvidere Mountain amphibolite, probably the youngest of the formations of Early Cambrian age. The Ottauquechee formation, composed of carbonaceous phyllite and quartzite, and phyllitic graywacke, is of Middle Cambrian age. The Stowe formation of Late Cambrian(?) and Early(?) Ordovician age overlies the Ottauquechee and is predominantly noncarbonaceous schist, though it also contains greenstone and carbonaceous schist and phyllite. The Umbrella Hill formation of Middle Ordovician age is characteristically a conglomerate in which the mineral chloritoid is common. The overlying Moretown formation, also of Middle Ordovician age, contains granulite and slate, also greenstone and amphibolite of the Coburn Hill volcanic member. The Shaw Mountain formation, made up of conglomerate, phyllite, and limestone, is the oldest Silurian unit. The Shaw Mountain formation is succeeded by the Northfield slate of Middle Silurian age. The igneous rocks of the region include various ultramafic plutonic rocks, such as dunite, peridotite, and serpentinite, probably of Late Ordovician age; sills and nearly concordant dikes of metagabbro of Late Ordovician age; biotite granite plutons or Middle or Late Devonian age, most notably on Eltey Mountain; and hypabyssallamprophyre, probably of late Permian age. Metamorphic zoning is shown by the distribution of rocks of the epidote-amphibolite facies and the greenschist facies in and near the Green Mountains, and near Coburn Hill and Eltey Mountain. Metasomatism related to regional metamorphism has produced porphyroblasts and quartz segregations in the sedimentary and volcanic rocks, and steatitization and carbonatization of serpentinite. Contact metamorphism has formed rocks of the epidote-amphibolite facies near granite plutons, and probably calc-silicate rock at the contacts of ultramafic plutons. The axial anticline of the Green Mountain anticlinorium and other anticlines and synclines to the east are the major longitudinal structural features of the area. These structures are complicated by transverse folds, particularly a syncline in the vicinity of Tillotson Peak. Early minor cross folds that are best developed in the Hazens Notch formation are believed to be genetically related to the transverse folds. The axial planes of the cross folds are folded about the axes of the later longitudinal folds of the Green Mountain anticlinorium. The longitudinal and transverse fold systems probably formed in the same episode of defor

Bulletin↗
USGS1959

Steatitization of serpentinite bodies in north-central Vermont

The talc deposits of north-central Vermont are associated with bodies of ultramafic rock, chiefly serpentinite, that are enclosed by metamorphosed sedimentary and volcanic rocks of Cambrian and Ordovician age; these stratified rocks form an essentially homoclinal succession in the east limb of the Green Mountain anticlinorium. The rocks at the localities described here are in the greenschist facies, and show only slight variations in grade of metamorphism between localities. The talc deposits consist typically of a core of serpentinite surrounded by successive shells of talc-carbonate rock and of steatite, next to which the country rock is altered for several inches to chlorite rock (blackwall). The steatite zone is rather uniformly about 3 feet thick, but the talc-carbonate rock and serpentinite zones vary considerably in thickness and are locally very irregular.

Vermont↗
USGS1951

Diamond-drill exploration of the Rousseau talc prospect, Cambridge, Vermont, and the Barnes Hill prospect, Waterbury, Vermont

The Geological Survey carried out a program of diamond-drilling at two localities in Vermont during the period July-September 1950. The deposits chosen for drilling were two of the more promising talc deposits associated with ultramafic rocks of several that had been mapped earlier by the Geological Survey, but could not be evaluated accurately because critical data were lacking that could be obtained only by sub-surface exploration. The Rousseau talc prospect is in Cambridge township, Lamoille County, Vermont. Surface rights are owned by L. and N. N. Porter; mineral rights are owned by Eastern Magnesia Talc Co. Three drill holes encountered no talc; the other five penetrated from 2 to 110 feet of talc ore. The deposit lies on the west limb of the Green Mountain anticlinorium. The country rock consists of quartz-chlorite-sericite schist and chlorite-albite schist. The schistosity strikes slightly west of north and dips 20° - 40° to the west. The talc deposit is roughly lenticular, and although generally concordant in detail with the schistosity of the country rock, is slightly cross-cutting in overall relationship. The entire deposit is composed of Tit (talc-carbonate rock) and steatite, out is presumably derived from ultramafic igneous rooks which were first serpentinized. The dimensions of the lens are about 700 feet by 500 feet by 130 feet. The talc is of good quality and is suitable, either as a mine run product or as a flotation concentrate, for most industrial uses other than industrial steatite. No pencil stock was encountered. The Barnes hill talc prospect is in Waterbury township, Washington County, Vermont. Surface rights are divided among Donald P. and Glenola Brown, John Barnes, and Eastern Magnesia Talc Co. One drill hole at Barnes hill was barren. The other six penetrated about 20 to 140 feet of talc ore. The deposit lies on the east limb of the Green Mountain anticlinorium. The country rock outside of the ultramafic rock body consists of quartz-chlorite-sericite schist, chlorite-albite schist; and chlorite amphibolite, which strike generally about N, 20° E. and dip steeply to the east. The ultramafic body is elliptical in plan, about 1,600 feet long and 360 feet wide. The vertical dimension is unknown. The original ultramafic rock has been almost completely serpentinized and is extensively altered to grit and steatite. The talc ore is somewhat irregularly distributed throughout the ultramafic body, but it is possible to delineate with considerable confidence portions of the body that consist predominantly of ore. The largest ore bodies are in the eastern and northern parts of the ultramafic body. The ore contains more or less admixed serpentine, and so is only of intermediate to fairly good color (whiteness). It is suitable for many industrial uses in which purity and high whiteness are not necessary. No pencil stock was seen.

Vermont↗