16 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



lion was given in two preliminary papers and the argument as a whole is here 

 presented for the first time. A summary on this highly complex subject is given 

 in the chapter. The origin of the thousands of feet of limestone and dolomite 

 found in the Rocky Mountain geosynclinal and in the Priest River terrane is 

 attributed to direct chemical precipitation on the floor of the open ocean. Statis- 

 tics show that the limestones of the earlier geological periods were originally 

 more magnesian than those of the later periods. This evolution of the limestones 

 is paralleled with a chemical evolution of the ocean. 



Chapter XXIV. — Is an introduction to a general theory of the igneous 

 rocks, the statement of which occupies the rest of the report. The Mode classifica- 

 tion is preferred and a table showing the average chemical composition of each 

 reck type is inserted. Magmatic heat in the earth is believed to be chiefly a 

 primitive inheritance, though some of it is due to radioactivity. The argument 

 for a general basaltic magma (perhaps highly rigid at the depth of the sub- 

 stratum) is presented, and is followed by the argument for a primary acid shell 

 at the earth's surface. All igneous action is preceded by abyssal injection, 

 whereby the basalt of the substratum mechanically displaces the lower part of 

 the earth's crust and rises to an average level which is at moderate depth below 

 the surface. A note on the essential mechanism of central-eruption volcanoes 

 as distinct from fissure-eruption volcanoes closes the chapter. 



Chapter XXV. — Discusses the classification of igneous intrusive bodies. 

 The favoured primary division is into injected and subjacent bodies, the former 

 group being largely satellitic to the subjacent masses, which are incomparably 

 the more important as to volume. 



Chapter XXVI. — The genetic problem of the eruptive rocks is, at its heart, 

 also the problem of the batholith. This chapter discusses the processes by which 

 batholiths are believed to have been formed. Their typical field and chemical 

 relations are sketched. The older hypotheses as to the methods of intrusion 

 are compared with the stoping-abyssal injection hypothesis. Abyssal assimila- 

 tion of sunken roof-blocks is a prominent element in the favoured explanation 

 of batholithic magmas. The chapter is largely a reprint of three preliminary 

 papers, the matter of which is here systematically assembled. 



Chapter XXVII. — Considers briefly certain points in the wide subject of 

 magmatic differentiation. The dominating control of gravity is emphasized. 



Chapter XXVIII. — The principles stated in the last four chapters are 

 here applied to a genetic classification of magmas, and then to rocks actually 

 found in the Forty-ninth Parallel section. The rock families specially discussed 

 are the granites, granodiorites, diorites, andesites, gabbros, basalts, comple- 

 mentary dikes, pegmatites, and the alkaline types, including the syenites. 



