R. A. Daly — Mechanics of Igneous Intrusion. 17 



Art. III. — The Mechanics of Igneous Intrusion r (Third 

 Paper;) by Reginald A. Daly, Massachusetts Institute of 

 Technology, Boston. 



Introduction. 



Hypothesis of niagrnatic stoping. 



Field relations of the typical batholith. 



Contact-shattering. 



Kelative densities of magma and xenolith. 



Sinking of the shattered blocks. 



Problem of the cover. 



Supply of tbe necessary heat ; niagrnatic superheat and its causes. 



Capacity of superheated, plutonic magma for melting and dissolving 



xenoliths. 

 Objection founded on rarity of evidences of assimilation at observed 



wall -rocks. 

 Abyssal assimilation. 

 Existence of basal stocks and batholiths. 

 Differentiation of the syntectic magma. 

 Origin of granite : the petrogenic cycle. 

 Origin of magmatic waters and gases. 

 Conclusion. 



Introduction. — In the April and August numbers of this 

 Journal in the year 1903, the writer published papers outlin- 

 ing the hypothesis of magmatic stoping as explanatory of the 

 rise of batholithic magmas in the earth's crust. The hypothesis 

 had taken form in his mind after some ten years of perplexity 

 as to the mode of intrusion which has actually characterized 

 granite bodies. In Vermont, New Hampshire, British Col- 

 umbia and other regions he had met with this urgent and 

 important field-problem. Everywhere the facts derived from 

 field observations were, in principle, the same ; the method of 

 intrusion seemed, for each batholith or stock, to be the same. 

 Since the writing of the two papers the writer has studied in 

 some detail a dozen other large batholiths and as many typ- 

 ical stocks occurring on the southern boundary of British Col- 

 umbia. For all of these also the stoping hypothesis appears 

 to afford the truest explanation of the mode of intrusion. 



Quite independently Barrell arrived at a similar hypoth- 

 esis, as he attacked, in 1901, the problem of the " Marysville 

 batholith*' in ^Montana. Unfortunately his monograph was 

 delayed in publication until 1907, so that it is only quite 

 recently that geologists have had the benefit of this brilliant and 

 thorough study of intrusive mechanism. f Barlow and Cole- 

 man have noted their belief in the efficiency of stoping as an 

 intrusive process.;): At the other side of the world, Andrews 

 has described the great intrusive masses of New South Wales, 



* Published by permission of the Commissioner for Canada. International 

 Boundarv Surveys. 



fU. S.^Geol. Surv.. Prof. Paper No. 57. 1907. 



X A. E. Barlow, Ann. Rep. Geol. Surv. of Canada, xiv, Part H, p. 79, 1904 ; 

 A. P. Coleman, Jour, of Geol., xv, p. 773, 1907. 

 Am. Jour. Sci.— Fourth Series, Vol. XXYI, No. 151.— July, 1908. 



9, 



