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Detailed gravity surveys were made in Camaguey Province, Cuba, as part of a systematic exploration program conducted by the U. S. Geological Survey on behalf of the General Services Administration to locate deposits of refractory‐grade chromite. During the period August 4, 1954, to April 5, 1956, a total of 41,921 gravity stations were established in nine areas embracing about 11.8 square miles in the Camaguey chromite district. The results of the surveys were used in connection with information obtained in geologic investigations to guide exploratory drilling. Gravimeters with low scale constants were used in making measurements sufficiently accurate to delimit anomalies as small as 0.5 gravity unit. The measurements were made over 20×40 and 30×60 meter grids and at stations 20 and 30 meters apart, respectively, along intermediate traverses in anomalous areas. Gravity differences were determined by single observations at individual stations, observing base stations hourly, and re‐observing a few stations to check drift and accuracy of measurement. About 11 percent of the stations established had to be re‐run because of errors in closure, microseisms, and instrument trouble. A large number of anomalies were found and evaluated according to geology, areal extent, and gravity relief. Depths to disturbing hypothetical bodies were computed to guide drilling. Test drilling of 106 positive anomalies revealed that ten features overlie deposits of chromite and 89 occur over bodies of other dense materials. The other seven anomalies were not explained by materials found in drilling. Core drilling on five of the chromite deposits revealed about 236,000 tons of chromite. An additional estimated 12,000 tons are contained in three deposits which were not blocked out. No estimate was made of the tonnage in two small deposits.

Seismic‐refraction measurements were made at 117 spreads along a line trending north from a shot point near Lamar, Colorado, to near Sidney, Nebraska, between April and August 1961. Crustal thickness from computation based on first and prominent secondary arrivals is about 48 km. The speed of compressional waves in the upper layers of the crust ranges from 2.9 (near surface) to 6.1 km/sec. Evidence was found for a crustal layer of compressional‐wave velocity 6.7 km/sec at a depth of about 28 km. The velocity of compressional waves in the upper‐mantle rocks is about 8.0 km/sec.

A seismic refraction survey was made in Mississippi along a line trending north from Ansley to Oxford. About 200 seismograms were recorded from 22 chemical explosions at 5 shot points spaced at intervals of about 50 krn between Ansley and Raleigh. The Salmon nuclear event, fired in the Tatum salt dome, was also recorded at several locations. A measured average velocity of 3.0 km/sec, to approximate all of the shallow sedimentary rocks, was used in depth calculations. The first strong refraction arrival represents a layer of 5.0 km/sec at a depth ranging from 3.1 to 3.7' kin, just under the unconformable boundary between the Upper and Lower Cretaceous sedimentary rocks. The crustal structure is complex below the Lower Cretaceous sedimentary rocks. A curved line could be fitted to the first arrivals between 15 and 150 km on each travel-time plot, increasing in apparent velocity from 4.9 to 6.9 kin/sec. All first arrivals between the 5.0-km/sec lower sedimentary rocks and the higher-velocity lower crust are included in one layer, giving an average velocity of 5.9 km/sec for 'basement.' The basement structure shows a low point corresponding to the trough of the Mississippi salt basin between the Tatum dome and Raleigh and a broad high feature cresting south of McNeill. First arrivals determine an intermediate layer in which the velocity is 6.9 kin/see, typical of basalt, and which rises from a depth of 19 km at Mc-Neill to 13 km near Raleigh. Strong events are apparently reflected from the M discontinuity at critical distances and beyond, and it is necessary to supplement meager first arrivals with the reflections to arrive at a delineation of the upper mantle. The average velocity in the upper mantle is 8.4 _ 0.3 km/sec, the average crustal thickness is 35 kin, and the M discontinuity dips 3 ø toward the south from Raleigh to Ansley. The complicated crustal structure may be due to the intermingling of two major geotectonic trends, the Appalachian and the Ouachita. Low to zero values of regional free-air gravity anomaly indicate that the region is near isostatic equilibrium. For our crustal model to be in isostatic equilibrium, the density must increase in the upper mantle below southern Mississippi. A hypothesis is made that increasing density in the mantle may be the mechanism of formation of the Gulf Coast geosyncline. 1963, June 1963, and September 1964. The profile consists of five shot points, spaced at intervals of about 50 km, and about 200 recording locations. The shot points, named after nearby towns and the Dribble site, are, from south to north, Ansley, McNeill, Dribble, Collins, and Raleigh. A total of 22 charges, each containing 500 to 20,000 lb of high explosive, were fired in drillholes. Seismograms were recorded at two levels of constant gain on both recording oscillographs and magnetic tape. Each recording spread consisted of six vertical-corn-ponent seismometers spaced over a length of about 2.5 km and two horizontal-component seismometers placed at one vertical-component seismometer location near ...

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