Three hundred and ninety-six new gravity observations form a reconnaissance survey of Puerto Rico with an average station spacing of about 2 miles in lines approximating nine north-south profiles and four east-west profiles.

A contour map of modified Bouguer anomalies shows positive values of about 90 milligals along the northern and southern coasts, increasing to about 165 milligals in the central part of the island. Utilizing this gravity information, with gravity and seismic information at sea in the area, a north-south structure section through San Juan is deduced.

The gravity anomalies are correlated briefly with surface geologic formations. The writers compare observed gravity anomalies across one section of the north Tertiary sedimentary basin with computed anomalies using reasonable density assumptions and sedimentary thicknesses deduced from geologic evidence.

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