758 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



pressure remaining quite unaltered, suffering merely a granulation with 

 the arrangement of the granulated material in parallel strings. This pro- 

 cess can be observed in all its stages, and there is reason to believe that it 

 has been brought about by pressure acting on rocks when they were deeply 

 buried and very hot. The anorthosite areas, of which there are about a 

 dozen of great extent with many of smaller size, are distributed along the 

 south and southeastern edge of the main Archean protaxis from Labrador 

 to Lake Champlain, occupying in this way a position similar to that of 

 volcanoes along the edge of our present continent.'* 



Gushing and Kemp have published somewhat detailed accounts of the 

 anorthosite forming a post-Grenville and pre-Cambrian body and its satellites 

 in New York State.f The mass covers about 3,000 square kilometres 

 in area. Cushing's petrographical descriptions show many points of agree- 

 ment with Adam's description of the still larger Canadian bodies. The 

 anorthosite generally crystallized with exceptionally coarse grain and a porphy- 

 ritic structure. Intense granulation is here again the rule, and from Cushing's 

 published 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 sviggest 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. :{: This idea may possibly explain the exist- 

 ence of the more pyroxenic phase regularly occurring inside the body. 

 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 bodies this 

 differentiation by fractional crystallization may have occurred just before the 

 huge masses were injected into the crust. 



Finally, the masses of anorthosite may represent enormous laccoliths, like 

 the Duluth gabbro as interpreted by Van Hise and Leith ; therewith lacking 

 most of the assimilative power of the bottomless batholiths. It is also not 



* F. D. Adams, Jour, of Geol., Vol. 1, p. 334, 1893. 



t H. P. Cushing, 18th Report of the State Geologist, Albany, 1900, p. 101; New 

 York State Museum Bulletin No. 95, 1905, p. 305. and Bull. 115, 1907, p. 471. J. F. 

 Kemp. 19th Ann. Report, U.S. Geol. Surv. Pt 3, 1899, p. 409 



t Op. cit., p. 417. 



§ As noted in a later section (nage 772), the same conception is adequate to 

 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-shells are due either to diffusion of basic molecules toward cooling sur- 

 faces, or to the combined influence of fractional crystallization and convection cur- 

 rents 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. 



