Doelter lays stress on the influence of high temperature, and especially of the highly heated gases in the igneous rock, in promoting corrosion of the cauldron-walls. He attributes greater power of corrosion to the magmas rich in silica, and agrees with R. A. Daly that the rapidly moving basic magmas reach the upper layers of the crust in a condition of comparative purity. Daly[73] may be looked on as an extremist in this matter; but it is hard for those who have studied regions where the deep-seated cauldrons have been cut across by denudation to avoid very large views of igneous absorption. The contact-zones between the igneous mass and the surrounding rocks are often seen merely in cross-section on the flanks of a batholite or laccolite. In the areas of Archæan rocks, on the other hand, where prolonged denudation has exposed the zones of repeated interaction over hundreds of square miles on an approximately horizontal surface, one may form some idea of the processes that are still effective in the depths. G. V. Hawes[74], in 1881, recognised the importance of the process known by the mining term of "stoping," as a means whereby igneous rocks work their way upward in the crust. Cracks in the overlying roof are entered by the magma, blocks are wedged off, and these are ultimately absorbed in the molten mass. In this matter Hawes stands as a pioneer. As the viscosity of the magma increases during cooling, the blocks last detached may remain embedded in the marginal zone. The remarkable purity of this zone, however, in many cases has raised an obvious difficulty; but it has been pointed out[75] that the modified marginal and composite rock may continuously sink down into the depths, aided by any of the causes that promote magmatic differentiation, while a fairly pure magma, almost of the original composition, is left on the crest of the advancing dome. R. A. Daly[76] has developed the stoping theory with considerable boldness. The areas most likely to carry conviction to those who doubt that igneous masses can be intruded at the expense of their surroundings are those where banded gneisses have arisen on a regional scale (see p. 160). THE RANGE OF COMPOSITION IN IGNEOUS ROCKS The broad division of igneous rocks into those of light colour and of low specific gravity on the one hand and those that are dark and heavy on the other is a very natural one, and Bunsen and Durocher insisted that two magmas were fundamental in the crust. In one of these, the "acid" magma, which gives rise to granites and rhyolites, silica formed about 70 per cent. by weight of the ultimate rocks; in the other, it formed about 50 per cent., and the products are basic diorites, gabbros, and basalts[77]. The former group of rocks is rich in alkalies, the latter, the "basic" group, in calcium, magnesium, and iron. The mixture of these more extreme types of magma was held to give rise to what are now called "intermediate" rocks.