766 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



abundant water found in obsidian and rhyolite is, in tbis view, largely or 

 wholly of secondary origin. Volcanic gases may similarly be largely ' resurgent ' 

 rather than ' juvenile.' In no case, however, would one class of emanations be 

 represented to the exclusion of the other. For post-Archean granites the emana- 

 tions are dominantly 'resurgent'; for gabbros the emanations are largely or 

 dominantly ' juvenile.' 



General Remarks on the Stoping Hypothesis. — The principal field-relation 

 on which the foregoing discussion hangs is the ' replacement ' of country-rock 

 by magma in the intrusion of stock or batholith. Slow digestion and solution on 

 main contacts has caused the replacement to a limited degree, but the facts of 

 nature seem to enforce belief in the more rapid and more important mechanical 

 replacement through magmatic stoping. 



The suggestion that batholithic magmas work their way up by stoping is 

 by no means new, and it is significant that, without any known exception, all 

 the authors advancing it have done so quite independently and as the result of 

 considerable field experience. Part of the idea was put forward by Kjerulf in 

 letter though not in spirit as early as 1879.* In 1894, Goodchild wrote: — ' Once 

 the rocks [the deeper-seated rocks] are reduced to the molten condition they 

 tend to eat their way upward and in any direction of least resistance — the 

 place of the material flowing upwards being at first taken chiefly by the colder 

 ui asses of rock, which sink within the magma as fast as they are quarried from 

 the sides of the vent.'f In 1896, LaWson mentioned the idea of the sinking of 

 blocks as a partial explanation of replacement. The statement was made in a 

 review and has been quoted in full in the present writer's first paper on the 

 mechanics of igneous intrusion (p. 2'83). 



A detailed study of the phenomenon as exhibited in the Elkhorn district, 

 Montana, was made by Barrell during the year 1900. His paper was withheld 

 from publication. From the manuscript he later published the following 

 summary : — 



' The contact [of the Elkhorn granite] is in its larger proportion a 

 broken and irregular surface slanting beneath a sedimentary cover, and it 

 is probable that at n,o great depth the granite underlies the greater part of 

 the district. If the granite merely broke through and involved the original 

 rocks of the area it now occupies, their entire absence from it as inclusions 

 is remarkable; if they had been carried away by fresh accessions from below 

 they should be found as inclusions in certain localities preserved from 

 erosion. On the supposition that they have sunk as fast as freed, the ab- 

 sence of inclusions may be readily explained. If, on the other hand, the 

 batholith were an intrusive mass of limited thickness, its bottom should 

 somewhere be exposed with the heaps of roof blocks resting upon it. As a 

 matter of fact, no indications of a bottom have been observed anywhere 

 within this batholithic area, and, although the evidence is negative in char- 



* T. Kjerulf, Udsigt over det sydlige Norges Geologi, Christiania, 1879. 

 t J. G. Goodchild, Geol. Magazine, 1894, p. 22. 



