THE INTRUSION OF LARGE BODIES OF IGNEOUS ROCK
Attention has been already called to the composite gneisses formed by the intrusion of an igneous magma between the leaves, as it were, of sediments. Such occurrences are often seen on the margins of batholites or of any kind of igneous dome, and they no doubt represent the picking off of layer after layer from the walls surrounding the intrusive mass. If these layers can become absorbed into the igneous rock, the crest of the dome can advance, and the dome itself can widen, so long as sufficient heat is supplied to it from below. Space is found for the intrusive mass at the expense of the marginal rocks; but it is obvious that the portions absorbed merely add to the bulk of the igneous material. The composition of the latter must also undergo modification. Its great size, reaching as it does far down into the crust, in comparison with the quantity of matter absorbed in the upper regions, may render such modification very difficult to trace beyond the latest zone of contact.
Petrologists differ very widely as to the extent to which igneous masses assume their place in the upper regions of the crust by processes of "stoping," absorption, and assimilation. The statement, however, in a recent work that "the assimilation hypothesis" is "still supported by some French geologists" is calculated to surprise those who recognise the trend of modern opinion both in America and on the continent of Europe. Far from the views of A. Michel Lévy, C. Barrois, and A. Lacroix, surviving as an expression of national perversity, they have been supported to a remarkable degree by the observations of Sederholm in Finland, of Lepsius and H. Credner in Saxony, of A. Lawson and F. D. Adams in North America, and by the careful reasoning of C. Doelter[69] based largely on his own experimental work. A. Harker[70] and J. P. Iddings[71] have argued that assimilation is merely a local phenomenon, of little importance in the theory of igneous intrusion. W. C. Brögger[72], however, who strongly supports the laccolitic view for the Christiania district, expresses himself with far more caution, and leaves the way clear for conclusions as to absorption and mingling of molten products in the lower regions of the crust.
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]).