In cross-sections showing granite batholiths,2 it is customary to depict them as extending down as far as the section goes, bounded by rather steep sides and with no known bottom, which is in accord with the definition of a batholith, but the question may be asked; how far is this due to those limitations of drawing which compel us, in showing what we know, to show also more than we know?
We find on geologic maps very large areas of a single color which indicate granite batholiths. If these were indeed all uniform granite they might be conceived as extensive, flat-lying sills of which only the top had been exposed by erosion, but it would be equally natural, indeed almost compulsory, to conceive them as sections of a uniform layer of granite “sial,” supposed to underlie the continents and transmit earthquake waves with a velocity of . . .