20 7?. A. Daly — Mechanics of Igneous Intrusion. 



tial to the idea of stoping per se. All of them may prove 

 incorrect without invalidating the stoping hypothesis in its 

 main featnre. Combining them and the idea of stoping, the 

 writer has constructed a general working hypothesis for the 

 origin of the igneous rocks. It seems, therefore, expedient 

 in the present paper to discuss the problem in its larger aspect. 



Field Relations of the typical batholith. — A principal fact 

 on which the stoping hypothesis is based has been amply illus- 

 trated in the published descriptions of granite stocks and 

 batholiths. Most, if not all, of these bodies in their accessi- 

 ble portions have replaced nearly equivalent volumes of the 

 respective country-rocks. They are generally cross-cutting 

 bodies. Their roofs are rough domes or arches, from which 

 large masses of the invaded rocks are sometimes pendant into 

 the crystallized granite. In each of many cases erosion has 

 destroyed much of the roof, and the roof -pendants, still pre- 

 serving the regional strike of their structure-planes, are to-day 

 exposed in section at the erosion-surface. Between the pend- 

 ants and between the main walls of a large batholith, hun- 

 dreds of cubic kilometers of country-rock formations are 

 plainly missing ; their place has just as plainly been taken by 

 the granite. 



A second principal fact is that, so far as granite batholiths 

 and stocks are known, each of these bodies shows a cross- 

 section enlarging with depth.* No one of them has yet 

 exhibited a floor composed of older formations. In relation to 

 visible country-rocks, all of them may be classed as subjacent, 

 rather than as injected, bodies. In relation to the wall-rocks 

 ten or more kilometers below the earth's surface, each batho- 

 lith may have been truly injected as a kind of gigantic dike, 

 but of this there is no direct proof. The actual observations in 

 the field show unequivocally that the batholitbic magmas have 

 worked their way up by replacing and absorbing the country- 

 rocks through the last few kilometers of ascent. Batholiths 

 are not laccoliths. 



A third generally observed fact is worthy of special atten- 

 tion. Where erosion has been profound the ground-plan sec- 

 tion of the typical stock or batholith is seen to be elliptical 

 and the profile-sections, as already noted, show that the upper- 

 contact surface of the intrusive is dome-shaped. Both in 

 ground-plan and in vertical sections the contact-surface is 

 relatively smooth. Apophysal offshoots do interrupt the wall- 

 rock, but the main-contact lines as mapped on ordinary geolog- 

 ical maps are characteristically flowing lines. Large-scale, 



*See the numerous sections of stocks and batholiths in Lepsius' " Geo- 

 logie von Deutschland " ; also Barrell's monograph cited, and the writer's 

 paper on the Okanagan Composite Batholith, Bull. Geol. Soc. America, xvii, 

 p. 330, 1906. 



