506 REGINALD A. DALY 



its chamber through channels which have fed the growing body 

 from larger, deeper-lying, generally invisible reservoirs. The cham- 

 ber is due to a parting of the country-rock into which the magma is 

 Ejected. An injected body is thus one which is entirely inclosed 

 within the invaded formations, except along the relatively narrow 

 openings to the chamber where the latter has been in communication 

 with the feeding reservoir. 



On the other hand, stocks, bosses, and batholiths never show 

 a true floor. They appear to communicate directly with their respec- 

 tive magma reservoirs. Each of these bodies shows field relations 

 suggesting that it is a part of its magma reservoir. The communica- 

 tion with the magmatic interior of the earth is not established by 

 narrow openings, but by a huge, downwardly enlarging opening 

 through the country-rock. In relation to the invaded formations a 

 stock, boss, or batholith is intrusive, but is subjacent rather than 

 injected. 



How a magma reservoir is enlarged by the volume represented 

 in the amount of intrusion signalized on the contacts of stock or 

 batholith is a matter permitting as yet of no absolute certainty. In 

 separating intrusive bodies into two primary divisions, one including 

 all injected bodies, the other including subjacent bodies, a classi- 

 fication will do good service in emphasizing the need of further 

 investigation into the mechanics of intrusion. No one has yet proved 

 that any granite mass over 200 square kilometers in area, and char- 

 acterized by vertical or outwardly sloping contact surfaces, is due to 

 injection. Whatever may be the probabilities, no one has yet proved 

 that such a mass has been intruded by any kind of assimilation of the 

 invaded formations. Some light has been shed on the origin of 

 batholiths and stocks, but they are certainly not understood as are 

 dikes and sills. 



So far as the method of intrusion is concerned, therefore, stocks, 

 bosses, and batholiths belong to a primary division of intrusive 

 bodies which may be defined as not demonstrably due to injection. 

 The principle is negative; it leaves the method of intrusion unstated, 

 but it brings into clear relief a principal contrast subsisting between 

 the greatest of intrusions, on the one hand, and dikes, sheets, lac- 

 coliths, etc., on the other. 



