428 Pirsson — Petrography of Tripyramid Mountain. 



mative one, since hornblende was present in large amount and 

 is therefore a critical mineral. The texture, though hue, is 

 megascopically granular. Hence the rock is a hornblende- 

 grano-andose. 



The classification of this rock, according to genetic qualitative 

 system, meets with difficulty. In the fact that it consists essen- 

 tially of brown hornblende and labradorite it at once suggests 

 camptonite, so common a dike rock in this petrographic prov- 

 ince. Reference to the table of analyses shows that it differs 

 considerably from the type given in No. IV. On the other 

 hand, this rock has a chemical correspondence with a dike rock 

 from the Belknap Mts., No. Ill, which was called by the author 

 spessartite, because in mineral composition, texture, and mode 

 of occurrence it agreed with that type as described by Rosen- 

 busch, although chemically it corresponded only in a general 

 way. From the genetic standpoint the Belknap rock, being a 

 member of a clan of alkalic rocks, could not be spessartite, 

 which should belong to the attendants of the common granite- 

 diorite set of magmas. In the present case, however, the 

 allegiance of the dike is less clear, but, all things considered, it 

 may, perhaps, best be termed a camptonite. ' It is of interest to 

 observe that this dike, and the previous one, furnish a good 

 example of how quite similar magmas may develop different 

 mineral compositions according to different local conditions. 

 The former consisted of augite, olivine, biotite, and plagioclase, 

 this one of brown hornblende and plagioclase. A difference 

 in the quantity and kind of mineralizing vapors may have 

 determined the formation, of hornblende, rather than the other 

 ferromagnesian minerals. 



General Petrology. 



From the standpoint of general petrology there is noth- 

 ing remarkable, or particularly novel, in the rocks of Tri- 

 pyramid, so far as the individual types are concerned. Their 

 interest centers chiefly in two features : the association which 

 the rocks present, and their origin in relation to the surround- 

 ing granite and gneisses. 



In regard to the association of types, this may be seen by the 

 mention of them : umptekite (alkalic syenite), quartz-syenite- 

 aplite, monzonite, norite, gabbro and camptonite. To the 

 uninitiated there may seem nothing remarkable in this assem- 

 blage, but to petrographers, who in these later years have been 

 considering the genetic relationships of rocks, especially as 

 they are found associated in differentiated complexes, it is 

 significant, and to many who hold very definite views on this 

 matter it may seem unnatural. For, if the syenite, mon- 

 zonite and camptonite come naturally together as an alkalic 



