4:2 7?. A. Daly — Mechanics of Igneous Intrusion. 



dishing and Kemp have published somewhat detailed 

 accounts of the anorthosite forming a post-Grenville and pre- 

 Cambrian batholith and its satellitic stocks in New York state.* 

 The batholith covers about 3000 square kilometers in area. 

 Cushing's petrographical descriptions show many points of 

 agreement with Adams's description of the yet vaster Canadian 

 batholiths. The anorthosite generally crystallized with excep- 

 tionally coarse grain and a porphyritic structure. Intense 

 granulation is here again the rule, and from Cushing's pub- 

 lished data it seems probable that the granulation followed 

 hard after the act of intrusion. The characteristics and field- 

 relations of the anorthosite are such as to suggest that they 

 have resulted from abyssal injections of magma which was not 

 superheated. A limited amount of stoping is possible in such 

 a magma but extensive assimilation of country-rock is not 

 possible for that magma. 



Kemp has suggested that the New York anorthosite has, 

 through fractional crystallization and the settlement of the 

 basic minerals of early generation, been derived from a normal 

 gabbro. f This idea may possibly explain the existence of the 

 more pyroxenic contact-phase regularly occurring in the bath- 

 olith. The contact rock is either gabbro or anorthosite-gabbro. 

 It may represent the original magma but little affected by the 

 settlement of the crystals of iron-ore, pyroxene and olivine. 

 In the more slowly cooled interior of the mass their settlement 

 could take place on a large scale.;}: In the Canadian batholiths 

 this differentiation by fractional crystallization may have 

 occurred just before the huge masses were injected into the 

 crust. 



The problem of the anorthosites is clearly as yet one for 

 speculation rather than one capable of final. solution. It seems 

 proper to believe, however, that, since all or nearly all of the 

 known anorthosite and gabbroid batholiths are of pre-Cambrian 

 age, they owe their origin to special pre-Cambrian conditions. 

 The stoping hypothesis as a whole expressly relates only to 

 conditions which have characterized orogenic belts in post- 

 Archean time. 



*H. P. Gushing, 18th Eeport of the State Geologist, Albany, p. 101, 1900; 

 New York State Museum Bulletin No. 95, p. 305, 1905, and Bull. 115, p. 471, 

 1907. J. F. Kemp, 19th Ann. Eeport, U. S. Geol. Surv. pt. 3, p. 409, 1899. 



fOp. cit., p. 417. 



% Incidentally it may be remarked that the same conception might con- 

 ceivably explain many internal basic contact-phases occurring in acid stocks 

 and batholiths. This explanation is evidently opposed in principle to the 

 prevailing view that the basic contact-shelis are due either to diffusion of 

 basic molecules toward cooling-surfaces, or to the combined influence of 

 fractional crystallization and convection-currents in the magma. Neither 

 of these hypotheses seems acceptable in the case of the anorthosite-gabbro 

 batholiths, and the writer has come to question their validity as final 

 explanations for some other types of intrusive bodies. 



