3^ NEW YORK STATE MUSEUM 



crystalline limestone dipping at a high angle into the lake. Several 

 dikes of later age cut all the rocks in the vicinity. Near the light- 

 house on the shore there are three dikes of great scientific interest. 1 

 The strike of the rocks varies greatly. The following measure- 

 ments were obtained: N 19 E, N 30 E, N 35 E (magnetic), 

 while the corresponding dips were 37 S, 69 E, 30 S, 6o° E, and 

 45° S 55 E. 



Away from the shore the Grenville rocks grade into syntectic 

 rocks because of the assimilation and soaking due to the igneous 

 activity of the intruding batholithic dark green gabbro ( Algoman in 

 age). This shell of syntectic rock 2 covers the gabbro so that the 

 typical rock is not exposed until the shoulders of Split Rock moun- 

 tain are reached. 



We may regard the entire point as a section of Grenville strata 

 which has been domed by an invading mass of gabbro. Numerous 

 pegmatites (quartz-andesine rock) of the gabbro cut the overlying 

 rocks. 



The wall rock of the pit is a mixture of various contact rocks, 

 including the usual green pyroxene rock with a high pyrite content. 

 The latter carries microscopic traces of gelena (PbS) which is 

 replacing the pyrite as beautiful intergrowths. The development of 

 the graphite is limited to the immediate vicinity of the pit, which led 

 the prospectors to think that they were dealing with a " vein." The 

 occurrence of graphite on the lake shore also caused them to believe 

 that they had found the continuance of the same ore body. The 

 incorrect use of the term " vein " has been extremely unfortunate 

 in practical mining, not only here but also in other sections of the 

 graphite area. 



After the miners had pushed their tunnel about 30 feet into the 

 hill they encountered a diabase dike (augite-camptonite) 10 inches 

 wide associated with a crush zone formed of brecciated fragments 

 of itself indicating post diabase faulting. The dike strikes N 70° E 

 with a dip of 45 ° N 20 ° W. Beyond the dike the rock is barren of 



1 Apparently a dike of diabase (microscopically an augite-camptonite) 

 originally 6 to 7 feet wide, fractured and intruded by another dike of similar 

 composition, but later in age. This was probably about 3 feet in width. Micro- 

 scopically, it comes near to hornblende-camptonite. This latter dike is split 

 and a third dike, 2 feet wide is intruded. This is the Bostonite. When Kemp 

 and Marsters visited this locality in their study of the dikes of the Champlain 

 region (U. S. G. S. Bui. 107) this " three story " dike was hidden by a boat- 

 house, but this has subsequently been removed, exposing the dikes. It is hoped 

 that this remarkable occurrence of three dikes cutting one another will be treated 

 more fully elsewhere. 



2 Meaning a composite rock due to a mixing of two or more of distinct species. 



