Pirsson and Rice — Geology of Tripyramid Mountain. 291 



ders are masses of coarse-grained gabbro which are succeeded 

 inwardly and above by monzonite and these by an inner core 

 of syenite. Where these are seen to meet there is a sharp transi- 

 tion line but only slight endomorphic evidence of contact. The 

 mass is surrounded with granite, but no contacts with this have 

 been found. It is characterized by a parting or sheet jointing, 

 common to all the rock types parallel to a dome surface. The 

 study has led to the conclusion that it is probably laccolithic 

 and that the different rock types have been formed by a com- 

 bination of differentiation and repeated upward movements of 

 magma, a process intermediate between differentiation in situ 

 and successive separated intrusions. Finally a rather full dis- 

 cussion of the bearing of the contacts and of the jointing on 

 various hypotheses is given, not alone for the purpose of elu- 

 cidating the origin and structure of the mountain, but also to 

 invite consideration and discussion of these as criteria in judg- 

 ing of the nature of complex intrusions. 



New Haven and Middletown, Conn., Dec. 1910. 



