174 GEOLOGY OF THE THmD DISTRICT. 



by well defined and regular vertical joints in two and often in three directions, resembling a 

 series of buttresses, only vertical in position, and therefore more properly pilasters, and giving 

 a good idea of cyclopean masonry. The view given in wood-cut 44 was taken at that cliff. 



26. ITHACA GROUP. 



49. 



No. 1. Intentiiate itrophomena (S. intentrialis). No.3. Ithaca strophomena (S. itliacensu). 



The rocks of this group are well exposed on Fall creek, on the Cascadilla, and in the other 

 water courses further south ; also in the quarries, and by the sides of the Inclined plane, where 

 considerable excavation was made, as well as in other points on the west side of that highly 

 interesting boundary of the district. They consist of a series of coarse hard shales and sand- 

 stone, the whole generally of a dark color, and without any observed definite arrangement of 

 the sandstone and shale. A few of the layers of sandstone are regular in their outline or form, 

 and resemble some of those which belong to the group below, as may be seen in the wall of 

 rock at Fall creek ; but this regularity of outline is rare, and no mineral character was observed 

 by which the group could be recognized, even at no very remote points from the line of that 

 section. 



The wall of rock at Ithaca cannot be less than about four hundred feet thick. It is a mass, 

 as before said, of hard and coarse shale and sandstone, dark in color, often brown after exposure, 

 owing probably to manganese. The particular order and kinds of mineral products were noted 

 at the Inclined plane, etc., but were too indefinite, and are therefore not given. The part which 

 is principally quarried near the village, as may be seen at McCormick's quarry, is a hard and 

 tough sandstone in irregular layers of a dark brown color when unaltered. The same mass 

 may also be seen at the excavation of the inclined plane, and on Fall creek at the tunnel. The 

 vertical joints in the rock in many parts are well exliibited, as for instance in the wall of the 

 same creek, on the east side above the falls. The joints are in two directions, and nearly 

 coincident with the cardinal points ; being a few degrees to the east of north for one set, and 

 about the same to the south of east for the other. The joints on the north side of the creek 

 have given origin to a series of gigantic pilasters, resembling in a measure those on the lake, 

 given in the wood-cut of the preceding group. 



