of this kind were first described in the Henry Mountains of Utah by G. K. Gilbert, who gave them the name of "stone-cisterns" or _laccoliths_, a word now commonly written _laccolites_. It may be questioned if the expansion of the gases in the intruding igneous rock is sufficient in itself to form the laccolitic dome. The igneous rock has probably been pressed into position by the forces that produced the earth-movements. In many cases, batholites seem to have worked their way upwards without any relation to earth-movements in the district. The processes by which they come into place among other rocks are worthy of separate consideration. 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.