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During the past 10 years, personnel of the U. S. Geological Survey have made about 14,000 observations of the Earth's gravity field in California, about 13,000 in Nevada, 3,400 in Oregon, and about 1,000 in the Hawaiian Islands. The total number of stations established in the four states is slightly in excess of 30,000.
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The late Tertiary volcanism in the San Juan Mountains of southwestern Colorado followed a long interval during which the crust was stable. The volcanic rocks have been divided into four main groups, (1) those of pre‐Potosi (?) age in the eastern area, (2) the Lake Fork quartz latite, San Juan tuff, and Silverton volcanic series, (3) the Potosi volcanic series and the Fischer quartz latite, and (4) the Hinsdale formation separated by long intervals without eruptions. Each of the groups is made up of rocks from basalt to rhyolite with chemical and other peculiarities. During the eruption of a group there was little deformation but there was subsidence after the eruption of two of the groups and doming after the eruption of the last group. The magma moved into the area from the sides and was erupted about as rapidly as it moved.in. For the group named the Potosi volcanic series, the first eruptions had a composition near that of the primary magma ‐ dark quartz latites. Their eruption was followed by a short time without eruptions during which the magma became layered by crystal settling. The next eruptions were rhyolites followed abruptly, probably because of active movement of magna by dark quartz latites. The process was repeated three times. Subsidence caused by withdrawal of magma followed, and after a long time a new and different magma moved into the area and yielded the rocks of the Hinsdale formation. After the Hinsdale, a magma intruded the area with subsequent doming, but without eruptions.
The specific yield of a water-bearing formation is defined as the ratio of (1) the volume of water, which after being saturated, it will yield by gravity to (2) its own volume (O. E. Meinzer, Outline of ground-water hydrology, U.S. Geol. Sur. Water-Supply Paper 494, p. 28, 1923). It is a measure of the quantity of water that a formation will yield when it is drained by lowering of the water-table. The determination of the specific yield is essential in many quantitative ground-water investigations. In 1931 a pumping-test was made in the Platte River Valley, in Nebraska, to determine by Thiem's method the permeability of the water-bearing sand and gravel that underlie the Valley. This test, which constitutes a part of a cooperative investigation by the Conservation and Survey Department of the University of Nebraska and the United States Geological Survey, was briefly described in the 1932 Transactions of the American Geophysical Union (p. 393). The well was pumped continuously for 48 hours, and measurements of the discharge were made every 30 minutes. During the period of pumping and after pumping stopped about 5,000 measurements were made of the depths to the water-surfaces in 80 observation-wells, which were located on lines radiating from the pumped well to distances up to 1,200 feet. Instrumental levels were run to the measuring points on each of the wells so that the altitudes of the water-surfaces could be determined for any time. From the data obtained in this test the average coefficient of permeability was computed to be about 700. In the present paper a method is presented for determining the specific yield of the water-bearing material from the data collected in this test, and the results of the computations are given. This method for determining specific yield was suggested by Meinzer (O. E. Meinzer, Outline of methods for estimating ground-water supplies, U.S. Geol. Sur. water-Supply Paper 638-c, p. 136, 1932).
Continued progress has been made in magnetic surveys over large sectional areas by G. R. MacCarthy. Beginning in Florida south of the Okeefenokee Swamp, magnetic traverses were measured and continued through the southern part of Georgia. The most northerly traverses crossed regions where they contacted the crystallines near the surface of the ground. All traverses were run in an east‐west direction. It is planned to continue the magnetic relief‐map described in the Transactions of 1939 of the American Geophysical Union by adding to it this portion of the State of Georgia.