262 J. Bar veil— ^Relations of Subjacent Igneous 



masses show no or almost no kinship in chemical composi- 

 tion with the surrounding walls. Such assimilation as 

 does take place must normally be abyssal, as the result 

 of the subsidence of blocks cut free by the roof dikes^ 

 the process which Daly has named stoping. Daly, 

 however, puts the emphasis of stoping on marginal 

 shattering and brecciation into fragments up to 10 meters 

 in diameter. The writer, on the other hand, looks upon 

 this as chiefly a decadent feature of magmatic action, 

 and regards the cutting free of large roof blocks by 

 intersecting dikes and sheets of high fluidity as the chief 

 feature which marks magmatic advance by stoping. 



The next point to be noted is that the batholiths quite 

 commonly reach their farthest limit before marked 

 differentiation begins to set in, as shown by the dioritic 

 rims chilled against their borders. Quite often, however, 

 the later granites break through to higher levels in the 

 crust, but such are often intrusive bodies, as great sheets, 

 dikes, or chonoliths, rather than broad batholiths. The 

 mode of batholithic rise at great depths can not be known 

 by direct observation, and what is here considered deals 

 only with the phenomena exposed by erosion. The roofs 

 of batholiths are greatly deformed, showing especially 

 domal and quaquaversal forms. This of course permits 

 greater upwellings of the magma, is distinct from stop- 

 ing, and simulates the results of laccolithic intrusion. 



How is the integrity of the roof preserved! In regions 

 of orogenic batholiths it would seem that a potent factor 

 must be the advance granitization in the cover. The 

 rising gases carry magma with them to the cooler regions 

 and will seek the stretching zones of the cover. Here 

 they precipitate their burden and pass on. Dikes 

 intruded high into the cover likewise will become chilled 

 and their flow tends to become checked as by the coag-ula- 

 tion of the blood in a wound. The precipitation of quartz 

 and feldspar by these means may go on to a large extent 

 and permit of adjustments of the cover to the hydraulic 

 and hydrostatic forces imposed from below. From time 

 to time come into operation also extraneous thrusting 

 forces, controlling the orogenic nature and modifying the 

 results which would be given by the rise and cooling of 

 the magma alone. 



In these relations of batholiths to covers, there is much 

 which is unknown, but the purpose here is not to follow 



