GEOLOGY OF MOUNT MARCY 5I 



Champlain valley, but the basic gabbros of the mountains had not yet 

 been recognized. To this latter group the dike undoubtedly belongs. 

 Prof. Ebenezer Emmons was entirely correct in regarding it as 

 igneous and in calling it a trap dike. It does, however, exhibit 

 effects of crushing and perhaps some recrystallization but it is too 

 basic and too rich in iron and magnesium to have been derived from 

 anorthosite. Undoubtedly it entered as a large basic dike. When 

 subsequently exposed at the surface its relatively easy weathering 

 produced the gorge. The dike strikes N. 50-60° W. Near the 

 lake it is 75 feet wide. Its dip is vertical. Professor Emmons states 

 that the dike can be traced up Mount Maclntyre across the lake. 

 The dike is peculiar in the amount of orthoclase which it contains. 

 There is enough to make one hesitate whether to class it with the 

 basic syenites or basic gabbros. This difficulty has been avoided 

 by calling it gabbro-syenite. No statistical measurements have been 

 made, but the rock is obviously much like the gabbros rich in ortho- 

 clase such as are described by Prof. W. J. Miller in the North Creek 

 area (Bulletin 170, page 29) and as mentioned on a previous page 

 of the present contribution. 



In 1895 in going from Avalanche Lodge to Lake Sanford by 

 way of the Indian pass the writer noted another dike of this same 

 character, nearly 3 miles to the northwest of Avalanche dike and 

 with a parallel strike and dip. As nearly as could be determined the 

 new exposure is just where the trail southwest from Clear lake 

 leaves the Mount Marcy quadrangle. If the location is correct, the 

 dike last discovered is somewhat north of the line of the strike of the 

 Avalanche dike ; but its course is in the same direction. Apparently 

 two intrusive masses entered the anorthosite along similar general 

 lines of weakness. 



Basaltic dikes. The last member of the hard rock formations con- 

 sists of the series of basaltic dikes which are so widespread in the 

 eastern Adirondacks. From fifteen to twenty have been discovered 

 in the Mount Marcy quadrangle, but there are undoubtedly others. 

 Almost without exception the observed dikes strike northeast and 

 southwest parallel with one set of the large structural lines of the 

 mountains. In most cases the dikes have found their way upward 

 along these lines of weakness. They are of later introduction than 

 the metamorphism of the ancient crystallines and may have entered 

 long after the Precambrian so far as any local evidence to the con- 

 trary is available. The acute observations of Professor Cushing^ in 



^ H. P. Gushing, " On the Existence of Precambrian and Postordovician 

 Trap Dikes in the Adirondacks." Trans. N. Y. Acad, of Sci., 15:2^52, 

 1896. 



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