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NEW YORK STATE MUSEUM 



involved with other gneisses, may indicate a greater age for 

 these. 



But the great similarity between the rocks of most of the 

 syenite masses seems to point to a close age relationship. The 

 gabbroid phase of the syenite would seem, like the gabbroic 

 borders of the anorthosite, to be due to differentiation after 

 reaching their present situations. The great similarity between 

 the two gabbroid rocks, as well as many minera logic resem- 

 blances between the ordinary anorthosites and syenites, would 

 be accounted for on the supposition that both rocks arose from 

 the differentiation of a common deep seated magma, the anortho- 

 site being erupted first and the syenite following at a somewhat 

 later date. Such phenomena as are presented by the syenitic 

 phase of the anorthosite, here appearing as a local differentia- 

 tion of the ordinary anorthosite, there occurring in dikes cutting 

 it, would be explained as, the one due to differentiation in place, 

 the other in the magma beneath, with the ascent of a slight 

 amount of material at this stage, following closely on the heels 

 of the main anorthosite intrusion. 



Granites. Perhaps the most abundant of all rocks in the 

 Adirondack region are gneissoid granites and granitic gneisses. 

 These are quite certainly of various ages. The granitic gneisses 

 associated with the Grenville rocks as well as those which make 

 up the bulk of the Saranac formation are unquestionably much 

 older than the anorthosite, as shown at contacts and also by 

 their occurrence as inclusions in the anorthosite. On the other 

 hand dikes of granite are not infrequently found cutting the 

 anorthosite, so frequently and over such a wide territory as to 

 argue the existence of considerable bodies of this rock whence 

 the dikes sprang. They are if anything still more frequent in 

 the syenite, in which small granite bosses appear as well. Also 

 localities are not uncommon in which two different granites are 

 found, the one cutting the other. It seems therefore likely that 

 all the granitic rocks of the region may be separated into two 

 great groups, an older and a younger, the former very gneissoid 

 in character and comprising the granitic content of the Grenville 

 and Saranac formations, and the latter much less gneissoid and 

 affiliated in age with the later great intrusions. In the latter 



