50 JR. A. Daly— Mechanics of Igneous Intrusion. 



in the less dense magmas, is seen to have become less serious 

 as that problem is viewed in the light of the new estimates of 

 magmatic density. The possibility that some of the Archean 

 batholiths were the scenes of actual, partial foundering of the 

 earth's crust had been noted ; the suggestion is made that, in 

 late pre-Cambrian time, it had become thick and strong enough 

 to inhibit extensive foundering of batholithic covers. 



Various objections to the hypothesis seem to fall away so 

 soon as they are confronted by the facts of experimental inves- 

 tigation on melted rocks and silicate mixtures. Other objec- 

 tions have been met by the facts derived from the field-work 

 of many observers. The facts of field-occurrence and field- 

 relations are opposed to the " laccolithic theory " and to that 

 of marginal assimilation ; on the other hand, these facts all 

 seem to be explicable on the stoping hypothesis, which, there- 

 fore, is taken by the writer to afford the best working basis 

 for the future investigation of granitic batholiths. In this 

 conclusion the writer is in full agreement with Andrews and 

 Barrell, two authorities who, with the intrusion-problem 

 expressly in mind, have carefully scrutinized actual batholiths. 



The hypothesis involves several important consequences, a 

 few of which have been considered. If magmatic stoping 

 and abyssal assimilation have largely operated during the 

 intrusion of post-Archean batholiths and stocks, it follows that 

 the material of these bodies is largely or wholly of secondary 

 origin. In each case it is a differentiate from a syntectic 

 magma formed by the solution of primary (acid) crust-rock or 

 of geosynclinal sediments in the (probably basic) magma of the 

 substratum. The order of eruption in batholithic areas, with 

 respect to the acidity of the rocks, need not be absolutely 

 fixed, but should show a strong tendency toward the succes- 

 sion of eruptives becoming more acid with decreasing age. 

 Lastly, since most post-Archean granites have replaced^ large 

 volumes of sedimentary rock, the suggestion seems warranted 

 that the water and other volatile matters regularly given off in 

 great volume from granitic magma, are also of secondary ori- 

 gin. Geosynclinal sediments are normally charged with rela- 

 tively abundant fluids ; it seems inevitable that these should, 

 in part at least, be given off during the solution of wall-rock 

 or engulfed xenoliths in an invading magma. 



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 stoj)ing. 



