The igneous rocks of Arkansas, especially those of Magnet Cove, have become classical through the careful and detailed work of the late J. F. Williams, whose volume* is, of course, well known to all petrographers. Study of his work, and especially of the map which he gives, led me to think that the structure of the mass and the relationships of the various rocks were not such as were briefly indicated by him, as will be shown later, but that the complex forms an excellent, though peculiar, example of a highly differentiated mass of magma, probably a laccolith, the rocks of which form a series of very interesting types.
It must be remembered that at the time Williams wrote his report the notion of laccoliths had not become widely accepted, especially in Germany, where he had studied. Furthermore, at this time none of . . .