REPORT OF THE CHIEF ASTRONOMER 763 



SESSIONAL PAPER No. 25a 



to the size and superheat of this body it will develop syntectic magma and, in the 

 end, freeze up. The next cycle opens with a new injection of basaltic magma. 



To discern the first and last products of a cycle among the actual forma- 

 tions of a province is evidently a difficult matter. In general, if two eruptive 

 masses are separated in time by several geological periods it is unsafe to regard 

 them as of one cycle. In compiling the foregoing illustrative list, therefore, 

 only those bodies, which by their evident consanguinity and by their relatively 

 close relation in age, have been considered. 



Furthermore, the list does not include the host of complementary dikes 

 often associated with batholiths and stocks. These are best regarded as pure- 

 differentiation products and afford no direct test of the general theory. 

 Even the sequence of the larger bodies noted in the list generally proves only a 

 successive differentiation in depth. The point here made is that the law of the 

 differentiation is the same as that necessitated by the stoping hypothesis 

 (gravitative differentiation of syntectics, increasing with time). So far, these 

 actual sequences all corroborate the hypothesis. In those cases, however, where 

 a younger batholith or stock replaces an older batholith, the splitting of the 

 syntectic formed by this assimilation may be expected normally to produce a 

 rock in the younger body which is less dense (generally higher in silica and 

 alkalies) than the older body. Such is the case with the Cathedral-Similkameen 

 combination, the Smelter-Cascade combination, and the granite-diorite group 

 of Sumas Mountain. The principle is further illustrated by many other 

 provinces, as in the diorite-syenite-granite sequence at Ascutney Mountain, 

 Vermont.* In fact, no eruptive sequence known to the writer, in any part of the 

 world, is of a nature opposed to the view that assimilation by primary basalt, 

 coupled with the principle of magmatic differentiation, is an essential con- 

 dition for the origin of magmas and of igneous rocks. 



Origin of Magmatic Water and Gases. — Finally, the stoping hypothesis 

 implies that, since post-Arehean batholiths have generally replaced large volumes 

 of sediments, the volatile matter which is normally trapped within a geosynclinal 

 prism should form an important part of the secondary magma. 



An approximate idea of the amount of volatile matter in the average 

 argillite,f sandstone and limestone of the world is readily obtained. For this 

 purpose we may use Clarke's composite analyses of 843 limestones, of 624 sand- 

 stones, of 27 Mesozoic and Cenozoic shales and of 51 Paleozoic shales, together 

 with 38 analyses of various argillites from different parts of the United States.:}: 

 From these analyses the writer has determined, for the argillites, the average 

 amount of water below 110° C. (H 2 0-), water above 110° C (H„0+), carbon 

 dioxide, carbon (and carbonaceous matter), and sulphur (in S0 3 ). These 

 averages represent, respectively, 116, 116, 106, 78, and 78 typical specimens of 

 argillite from as many localities. The averages for sandstone and limestone 



* Bull. 209, U.S. Geol. Survey, 1903. 



t The term ' argillite ' here includes both shales and slates. 



X F. W. Clarke, Bull. No. 228, U.S. Geol. Surv., 1904, p. 20. 



