SIMA AND SIAL
It has long been known to geologists that below the granitic rocks, which are very widely, spread over the continental surfaces, there is an even more widely spread, and probably universal, layer of basaltic rock. The evidence on which this knowledge is based consists of (1) the innumerable dikes of diabase which cut the granites in all parts of the world, and (2) the universality of basaltic volcanoes. The dikes, such for example as are so common in the Archean terranes of Canada, are narrow, usually not exceeding 150 feet in width. Individual dikes can rarely be followed for more than a few miles; but zones of dikes continue in the same general course for scores of miles. The dikes occupy vertical gashes in the granites, which are probably large-scale shrinkage cracks due to loss of volume by cooling. The dikes were . . .