320 



NEW YORK STATE MUSEUM 



Three miles north of the depot ai Tupper lake there is a small 

 rock cut in a gabbroic-looking rock, somewhat more feldspathie 

 than the rock of the two dikes, but very similar nevertheless and 

 plainly closely related to them. It contains some 30$ of dark 

 minerals, and its feldspar is all of intergrowth types. Its field 

 relations are with the syenite as a border phase, though it is 

 close to the anorthosite boundary, and both its mineralogy and 

 its chemical analysis show it to be a somewhat basic syenite. 



In a cut a mile farther north anorthosite gabbro appears 

 which shows rather frequent labradorite augen. but whose 

 granular feldspars are andesin and microperthite in about 

 equal quantity. In other words, the rock is an anorthosite with 

 syenitic tendencies. 



About halfway between Tupper Lake village and Wawbeek an 

 interesting glaciated rock surface is shown by the roadside near 

 the town line, exhibiting anorthosite gabbro cut intrusively by 

 syenite, not as a single dike but as an invasion in force, wedging 

 apart and surrounding great horses of the anorthosite. This 

 syenite shows numerous feldspar augen and much more strongly 

 resembles the usual syenite than do the preceding rocks. It is 

 however more basic than the normal rock, having a considerable 

 pyroxene-hornblende-garnet content. Feldspar forms some 75% 

 of the rock however and is nearly all microperthite. 



Nearly 1 mile farther east and hence that much farther within 

 the anorthosite mass, is a knoll of gabbroid syenite almost pre- 

 cisely like the Colby pond dike. Its field relations to the sur- 

 rounding anorthosite are not exhibited, though from analogy it 

 must be a very large dike or else a small boss. 



The small anorthosite outlier in Litchfield park has been 

 already referred to. It is all surrounded by syenite gneisses and 

 with good contacts exposed on one side. The rock is much more 

 acid than in the previous exposures and yet is not normal 

 syenite, though it is an igneous rock, a syenite, and identical 

 with what appear as phases of the normal syenite elsewhere. 

 It becomes fine grained at the contact, while the anorthosite 

 shows no change in grain, and seems quite conclusively the 

 younger rock; hence the disposition to regard the anorthosite 

 as an inclusion in the other. 



