1. Structure and Petrography of Some Diabase Dikes in Central South Carolina PRIVETT, DONALD R., Dept. Geology, University of Tennessee, Knoxville, Tenn. Diabase dikes of questionable Triassic age intrude metamorphic rocks and granites in the Piedmont physiographic province north and west of Columbia, South Carolina. Deep chemical weathering prevents accurate determination of the size and orientation of the dikes. In general, they strike 20--40 NW., and at two outcrops in roadcuts dip steeply 80-90 NE. Widths range from 2 to 300 feet, averaging 30 to 50 feet, whereas lengths range from about 300 feet up to 7 miles. The dike rocks are olivine diabases with ophitic-to-intergranular texture. Major mineral modes range from 40-60 per cent plagioclase (An61), 13-36 per cent augite (Ca43Mg46Fe11), 10-30 percent olivine (Fa31), and 1-5 per cent magnetite-ilmenite. Accessory and secondary minerals present are tremolite-actinolite, apatite, biotite, hornblende, and leucoxene. Plagioclase is generally unaltered and occurs both as phenocrysts and interstitial laths. Augite is altered to uralite along cracks and gtrain boundaries, and olivine is always partly, sometimes completely, altered to serpentine minerals and magnetite. Two dikes sampled at measured intervals from contact to contact showed modal increases in plagioclase from peripheries to centers, whereas olivine-plus-pyroxene decreased. Olivine modal percentage varies inversely with pyroxene mode. However, when considered separately, the olivine-pyroxene modes fluctuate, showing random variation across the dikes. No systematic transverse variation in the compositions of the major minerals was noted.