Discussion: The Disintegration and Exfoliation of Granite in Egypt

Abstract

In his interesting description of the disintegration and exfoliation of granite monuments in Egypt, Donald Barton' has clearly demonstrated that the exfoliation has developed in historic time and only on surfaces exposed to moisture. He concludes (p. III) that "this exfoliation in Egypt cannot be even partly hypogene" and that "much of the extensive known disintegration and exfoliation of granite in the moist temperate regions of the world is not a result of unloading of the rock but an effect of moisture, presumably hydration plus incipient further alteration." If speed of development of exfoliation is important, the rapid exfoliation in Egypt may be matched by an instance of even more rapid exfoliation after unloading, called to my attention by W. Armstrong Price,2 who described the expansion and exfoliation of cores of Tertiary sandstone and shale which developed when they were brought to the surface from oil wells drilled in Texas. He first noted the structure in cores taken from 2,500 feet below the surface. The large-scale exfoliation of glaciated surfaces of the Sierra Nevada granite has been ascribed to dilation after unloading,3 and it has developed nearly as rapidly and much more abundantly than the exfoliation in Egypt. Rock exfoliation has developed in widely diverse geologic environments, and many processes4 appear to have been involved. Among them, hydration seems to have been prominent, but possibly each