PERIDOTITE DIKES NEAR ITHACA, N. Y. 



GEORGE C. MATSON 

 Chicago 



The country rock at Ithaca consists of Upper Devonian shales 

 and sandstones. The rocks are traversed by two series of well- 

 marked joint planes. One series extends nearly north and south; 

 the other series, east and west. Both series are at right angles to 

 the bedding planes. In general, the joints are more abundant in 

 the shales than in the sandstones. 



Many dikes have been discovered in the north-south joint planes, 

 but none have been found in the east-west joint planes. This fact 

 may be accounted for in two ways. The streams, which usually flow 

 either east or west, cross many more north-south than east-west 

 joint planes. Therefore the dikes in the former are more likely to 

 be exposed than those in the latter. However, as the dikes were 

 probably intruded while the rocks were being folded, it seems more 

 probable that the east- west joint planes, which are roughly parallel 

 to the folds, were kept closed by compression during the period of 

 intrusion. 



Location and size of the dikes. — The existence of a few dikes near 

 Ithaca has long been known. Professor J. F. Kemp 1 speaks of the 

 three dikes in Cascadilla Creek near the bridge at the entrance to 

 the Cornell campus, and he also mentions the one in Six- Mile Creek 

 above Green Tree Falls. The dike just above the upper bridge 

 across Cascadilla Creek measures about 3 feet. It outcrops on the 

 south side of the gorge ; on the opposite side the gorge wall is covered 

 with debris. The other two dikes are each about 2 inches wide. 

 They show on the south wall of the gorge between the upper and 

 lower bridges. 



In his Geology 0} the Third District of New York, Vanuxem 2 



1 J. F. Kemp, "Peridotite Dikes in the Portage Sandstones Near Ithaca, N. Y.," 

 American Journal of Science, 3d Ser., Vol. XLII (1891), pp. 410-12. 



2 Vanuxem, Geology of the Third District of New York (1842), p. 169. 



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