B. A. Daly — Mechanics of Igneous Intrusion. 287 



Abyssal assimilation. — Other tests of the hypothesis in 

 addition to those of direct laboratory experiment and field 

 observation, are obviously suggested when we leave the igneous 

 contact and consider the fate of the submerged blocks. 



Assuming that overhead stoping is competent to form in 

 the earth's crust chambers of the size represented in stocks and 

 u batholiths," the inquiry leads perforce into the problem of 

 magma genesis. If so much mechanical loss has been suffered 

 by the formations invaded by a given magma, what effect will 

 be wrought on the composition of that magma by the sub- 

 mergence of the corresponding fragmentary material ? How 

 far will such blocks sink? What is their future history? If 

 it can be shown that the foreign material is to a magma what 

 food is to an animal, does it follow that there will be the 

 equivalent of digestion and, by diffusion, still more intimate 

 assimilation of the -country-rocks by the magma? At this 

 point, we must enter the region of theory on the origin of 

 all igneous magmas, a region certainly beset with great 

 impediments to progress. Only so far as these general con- 

 siderations seem to call for immediate treatment as a further 

 test of our hypothesis, and then but in skeleton outline, does it 

 seem advisable to follow out the subject at this time. It is 

 felt that no other special apology is needed for thus venturing 

 into what must always remain speculation to so large an extent, 

 for any theory of intrusion must be similarly correlated with 

 the chemical and physical characters of igneous rocks. 



As experiment has thrown much light on the mechanics of 

 intrusion, it may also help deductive reasoning in determining 

 the probable future of submerged blocks. Whether any block 

 would be permitted to sink thousands of feet or several miles 

 from its former niche in the vault, it is probably impossible to 

 say. The locus of denser, though still liquid, layers of magma 

 in which the block would float, cannot be foretold. The block 

 might even sink to the deep level of pressure-solid magma, if it 

 be not already digested on its downward journey. In either 

 case it must undergo an increase of pressure and, with the 

 greatest probability, an increase of temperature. 



The added pressure would have, according to the experi- 

 ments and field studies of Barus, Doelter, Daubree, Fouque, 

 Michel Levy and others, the secondary effect of increasing the 

 capacity of the magma in retaining water and other solvents, 

 even at very high temperatures.* So important are other 

 experiments in this connection that a brief resume of certain 

 results accruing from them must be given. 



* Among the more recent papers, cf . : C. Barus, this Journal, xxxviii, 408, 

 (1889) and xli, 110 (1891). C. Doelter, Centralbl. f. Min., etc., 1902, p. 550 ; 

 and Tscher. Min. u. Petrog. Mitth., xxi, 218 (1902). 



