GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



319 



of the two. so the evidence of age relations between syenite and 

 anorthosite must be sought from those masses of the syenite which 

 adjoin anorthosite. The only two such masses in the north- 

 ern Adirondacks, aside from the small anorthosite outlier in Litch- 

 field park, Franklin county, are the Tupper lake and Saranac river 

 syenites. Each of these has furnished some evidence. 



A long cut on the Saranac branch of the New York Central and 

 Hudson River Railroad near Colby pond exposes an apparent dike 

 of a gabbroic-looking rock some 30 feet in width, in the midst of 

 the anorthosite gabbro of the cut. The dike show T s a heavy black- 

 ish rock, darker colored and finer grained than the anorthosite 

 gabbro. The thin section shows that its affiliations are with the 

 syenites, and that it is quite like the gabbroic phase of the Diana 

 syenite. It holds some 35$ to 40^ of minerals other than feldspar. 

 These being augite, hypersthene, hornblende, biotite, garnet, mag- 

 netite and quartz (with small amounts of zircon, apatite, titanite 

 and pyrite). The feldspar is entirely of intergrowth types, fine 

 microperthitic or miorographic intergrowths of orthoclase and 

 albite (or oligoclase). Quartz makes some o$ of the rock. The 

 nature of the feldspar makes reference of the rock to gabbro im- 

 possible, yet it looks exceedingly like the ordinary dark gabbros 

 of the region and is very difficult to tell from them in the field. 



The west wall of the dike is well shown and is sharp, so that 

 there seems no doubt that it actually is a dike. The igneous 

 nature of the rock is beyond question. 



Since the anorthosites grade at times at their borders into gab- 

 broic gneisses which positively can not be distinguished from these 

 gabbroid syenites in the field, it is evident that boundary mapping 

 is attended with considerable hazard in districts where the two 

 rocks adjoin and both show this differentiation. 



A similar dike, 8 feet wide, is found cutting anorthosite in a 

 railroad cut 3% miles west of Saranac Inn station. The main 

 difference between the two rocks is that in this dike the feldspar, 

 instead of consisting entirely of intergrowth types, as in the pre- 

 vious case, shows quite a considerable percentage of andesin. 

 though the microperthite largely predominates. The rock is by 

 no means so distinctly a syenite as in the previous case, but is 

 rather an intermediate rock. 



