Petrographic, chemical (11 rock analyses), and age determinations (7 whole-rock K-Ar) were made on 27 "basement" well samples from 22 Florida counties north of Lake Okeechobee, and 3 additional K-Ar determinations were made on samples from Georgia. Most of the specimens are diabase, basalt, rhyolite, and basaltic and rhyolitic ash, generally fresh and unaltered, although contiguous sedimentary rocks may show considerable alteration. The igneous rocks are relatively young, the youngest age determined being 89.3 ± 2.2 m.y. (Late Cretaceous). Metamorphosed "granite" and "diorite" which may be altered arkosic sedimentary or igneous rocks gave older ages, up to 480 to 570(?) m.y. (Early Ordovician to Early[?] Cambrian). These older ages were found in two metamorphosed roc s from the central east coast of Florida, where other granitic metamorphic rocks have been reported; similar rock (undated) is present on the central west coast of Florida, and a Paleozoic age (middle Carboniferous) also was found for a metamorphic rock (hornblendite) from western Georgia. These older dated rocks have been considered to be part of a metamorphic basement intruded by the younger igneous rocks; however, hydrothermal alteration of igneous rocks may produce metamorphic textures, and sedimentary rocks may be "granitized" by contiguous igneous intrusives. No unambiguous Piedmont-type crystalline metamorphic rocks, such as mica schist, garnet schist, or sillimanite-andalusite-kyanite schist, have been found by drilling in Florida.

The chemical analyses of these rocks per se have little correlative value because of the small size of available samples and the undoubted variability of the rock bodies represented. However, they apparently are the first and only such analyses made of Florida igneous-metamorphic rocks.