Pirsson — Petrography of Tripyramid Mountain. 431 



by juvenile gases. Neither author offers a discussion of the 

 chemical aspects of the case, the hypotheses being founded on 

 geological and mineralogical occurrences and associations. 



It would be out of place to discuss these views in this place, 

 but it may be pointed out that they can scarcely be appealed 

 to for an explanation of the alkalic syenite of Tripyramid. 

 For, since the magma was ejected through and into a granite 

 bathylith with gneisses and acid schists above, there could have 

 been neither saline beds nor carbonate masses operative upon 

 it. It might be suggested that the syenite was formed by the 

 action of such acid rocks on the monzonite or gabbro, but this 

 has already been considered in the former paper. In this 

 respect Tripyramid is like Red Hill and the Belknap Mts., 

 occurrences of alkalic rocks to the southward, which have been 

 studied and, in part, described by the writer.* In them also 

 the intrusions have been through and into gneisses and schists 

 and there is no evidence of saline beds or carbonate rocks 

 having taken part. These cases seem to fall under the alterna- 

 tive view suggested by Daly,f and it therefore appears that 

 some alkalic magmas and rocks are formed without the aid of 

 particular sediments. 



It has been suggested that the border zones of concentric 

 masses have been produced by absorption of the enclosing 

 rocks. This could not have been the case at Tripyramid, for it 

 is clear that the gabbro could not have been made from the 

 monzonite magma by the absorption of either granite, gneiss, 

 or schist. It must be regarded either as a separate intrusion, 

 or as a differentiation product. 



Summary. 



This article deals with the rocks of Tripyramid Mountain, 

 which is shown to consist of alkalic syenite (umptekite), mon- 

 zonite and gabbro with associated dikes of quartz-syenite aplite 

 and lamprophyres allied to camptonite. These various types 

 are described and, in most cases, chemical analyses of them 

 given. The latter show in the case of the basic dikes that 

 magmas of similar composition may produce rocks mineralog- 

 ically different. The systematic positions of the various types 

 are also discussed and in conclusion the bearing of the facts 

 observed on the subjects of genetic classification, and on the 

 origin of alkalic rocks, is treated. 



Sheffield Scientific School of Yale University, 

 New Haven, Dec. 1910. 



*This Jour., xx, p. 344; xxii, pp. 439, 493, 1905-1906 ; xxiii, pp.257, 433. 

 f Op. cit. p. 113. 



