ABSTRACTS AND DISCUSSIONS OF PAPERS 105 



This may seem at first sight to be an argument against the validity of other 

 modes of intrusion in other regions, more especially against the validity of the 

 stoping hypothesis. Although, as Daly has said, the stoping hypothesis was 

 postulated independently by a number of geologists, it has been so largely 

 developed by his work that he is deservedly regarded as its chief founder and 

 expositor. As one of those, however, who have contributed papers in which 

 it is held that the rise of certain batholiths was by stoping, it seems appro- 

 priate for me to speak of the relations of the two modes of intrusion. The 

 invasion of the Black Hills granite by crustal distension seems a logical con- 

 clusion, but at the same time does not controvert the evidence that certain 

 other bodies have risen to their final boundaries by a process of stoping. In 

 this statement I am not taking exception to Mr. Paige's conclusions. In fact, 

 we have discussed this topic and I think we agree; but where a more or less 

 novel view, such as the stoping hypothesis was ten years ago, is advocated as 

 an explanation of certain examples, the idea is apt to arise among readers 

 that the expounders wish to give it a universality of application and eliminate 

 competing hypotheses. 



What, then, is the adjustment of factors which in one case may lead to 

 lateral displacement as a chief mode of invasion, in another case to overhead 

 stoping, permitting a passive rise? The rise of magmas from abyssal depths 

 would appear to result from the unbalancing of a hydrostatic equilibrium. 

 The liquid column is lighter than the surrounding rocks. As the liquid arises 

 above that datum plane where the liquid and solid are under the same pressure, 

 the internal pressure at any level is diminished by the weight of the column 

 of liquid below ; the pressure in the surrounding rocks is diminished by the 

 greater weight of the rock column between the level and the datum plane 

 below. A bursting and intruding pressure is consequently developed. In the 

 zone of flowage the thick cover would permit this internal pressure to act 

 laterally, pushing the walls aside. Injection into the roof in steep foliation 

 planes also implies a lateral distension. More or less doming of the cover is 

 of course also to be postulated. 



When a large magmatic body has advanced, however, comparatively near to 

 the surface, a lateral distension of the walls or cover becomes subordinate, 

 because the line of least resistance is now for the magma to dome the cover 

 upward, to produce intrusion fractures, and to intrude in distinct sheets and 

 dikes, rather than to produce a Ut-par-Ut injection. The mechanical conditions 

 making stoping a dominant process are then found especially in the zone of 

 fracture ; those making for injection, mashing, and crystallization with lateral 

 distension of the wall rocks prevail in the zone of flow. The Black Hills 

 granite belongs to the Precambrian. In these ancient batholiths crustal dis- 

 tension by the invading magmas appears to have been a dominant process, 

 though stoping even here may have participated. The far younger batholiths 

 of the Cordillera were intruded to high levels and with more or less absence 

 of compressive forces. In these stoping appears to be a dominant process in 

 the higher stages of their invasion. If we could restore the great depths 

 eroded from the Archeozoic and look at the former higher levels, or if we 

 could look deeper into the crust to observe the relations of post- Jurassic 

 batholiths yet concealed, the distinction in dominant modes of intrusion evi- 

 dent between the older and younger periods of igneous activity might be found 

 to have largely disappeared. 



