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NEW YORK STATE MUSEUM 



Clinton county brougfht out as early as 1896 the existence of two 

 series of basic intrusions, whereas before this time no such con- 

 trasted grouping had prevailed. His field work along the edge of 

 the Potsdam sandstone which rested on the older gneiss revealed 

 many dikes of diabasic character in the gneiss, but none passing 

 upward into the Potsdam. On the other hand we knew of trachytic 

 (or bostonite) dikes and rare basaltic types (camptonite and others) 

 which cut both ancient crystallines and Paleozoic strata up through 

 the Utica slate. 



The basaltic dikes in the Mount Marcy quadrangle are dense, 

 finely crystalline rocks whose exact mineralogy and textures can be 

 determined only with the microscope. Slides have not been prepared 

 of every dike met and one can only say that to the unaided eye they 

 seem in the cases from which no slides have been studied to be 

 identical with the Precambrian diabases. On the other hand micro- 

 scopic examination of a dike in the Jolins Brook valley, just below 

 the entrance of Ore Bed brook, shows it to be related to the campton- 

 ites. It consists of an interlacing mass of minute augite prisms about 

 0.05 mm broad by 0.5 mm long with many bits of magnetite in a 

 matrix apparently plagioclase. The twinning of the plagioclase is 

 often apparent, but is not always pronounced. There may be some 

 other minerals, such as analcite, involved. The texture is exces- 

 sively fine and with high powers the identity of the minerals is 

 difficult to establish. Of much the same general character is the 

 basaltic dike, 5 feet wide, shown in figure i and illustrated in plate 

 21. The phenocrysts are well bounded and usually zonal augites, 

 brown, basaltic hornblendes and olivine. The ground mass is chiefly 

 an interlaced aggregate of minute prisms of augite and hornblende, 

 with some lighter colored minerals, presumably plagioclase and an- 

 alcite or some similar alkaline-alumina silicate. With such small 

 components it is difficult to get satisfactory tests in a naturally 

 opaque rock. Magnetite in small irregular bits does not fail. 



Clearly these dikes, so far as microscopically studied, are camp- 

 tonites and are related to the post-Ordovician dikes of the Cham- 

 plain valley. This is a peculiar feature and may stamp them as a 

 local outbreak of the same magma far in the mountains. The dis- 

 covery of a fragment of a bostonite dike, although not in place, yet 

 penetrating the rock regarded as Grenville gneiss partly digested in 

 anorthosite, and near the headwaters of Johns brook, is another 

 peculiar and corroborative observation. The camptonites and bos- 

 tonites are associated in the Champlain valley. There is every reason 



