430 Pirsson — Petrography of Tripyramid Mountain. 



The geology of Tripyramid, as it is now known, indicates 

 that a body of monzonitic magma, as this appears to be the 

 main rock mass, after, or during intrusion, has so differentiated 

 that the outer part has developed into gabbro and an inner 

 part into alkalic syenite. This does not seem illogical, for the 

 intermediate position of monzonite with respect to alkalic and 

 sub-alkalic rocks has long been recognized, and its chemical 

 similarity to the generalized earth magma of Clarke and "Wash- 

 ington clearly perceived. But if a monzonite magma is thus 

 really intermediate, it cannot be expected to always differentiate 

 in one direction alone, to the alkalic side. From it we must 

 expect to get intermediate series, running in various directions 

 and connecting the alkalic with the sub-alkalic rocks. Thus 



Alk. Granite — Alk. Syen. — Foyaite— Essexite — Shonkinite, etc. 

 Monzonite 



Common Gran. Quartz Diorite — Diorite — Gabbro — Peridotite 



in the present case our series runs diagonally as shown above. 

 The norite is simply a phase between the monzonite and 

 gabbro. It seems to realize the idea expressed by Cross in 

 saying : " Given an intermediate monzonitic magma, is it not 

 natural to suppose that its descendent magmas must be inter- 

 mediate in many respects between the series from foyaitic and 

 dioritic parent magmas and that a shifting of conditions may 

 throw the dominance of characters one way or the other ? "* 

 Considered from this point of view, the two dike rocks pre- 

 viously described would be merely products of the monzonite 

 magma, whose differentiation paths were towards essexite, 

 rather than towards the gabbro, and whose complementary 

 derivatives are to be seen in the quartz-syenite aplites. 



The other interesting feature of these rocks is their origin 

 with respect to the surrounding older ones. Recently the view 

 has been put forward that the alkalic rocks are due to the 

 fusion and absorption of particular kinds of sediments by sub- 

 alkalic magmas. Jensenf regards these sediments as having 

 been Archean saline beds, while Daly, J on the other hand, 

 considers them to have been masses of carbonate rocks, and 

 that juvenile carbonic acid assisted in the process. The latter 

 also suggests that some alkalic rocks may have been formed by 

 simple differentiation, by " splitting" of the magmas assisted 



* Op. cit. p. 484. 



f Distribution, Origin and Eelationships of Alk. Rocks, Proc. Linn. Soc. 

 N. S. Wales, xxxiii, p. 491, 1908. 



X Origin of the Alk. Rocks, Bull. Geol. Soc. Amer., xxi, p. 87, 1910. 



