J. F. Kemp — Great Shear-zone in the Adirondacks. Ill 



lake it is 75 feet across. The walls are vertical. The so-called 

 dike terminates in the lake where it is doubtless cut off by the 

 cross fault that outlined the pass. It strikes N. 50-60 W. 



The general rock of the mountains is a lighter colored variety 

 of the plagioclase rock which is commonly but not always cor- 

 rectly called norite. Sections from the wall of the pass show 

 'coarse plagioclase in much the largest part, with which is prob- 

 ably a little orthoclase. The feldspar contains many minute 

 inclusions which are probably small crystals of the dark silicates 

 and iron ores, exhibiting thus a common character of the feld- 

 spars of rocks belonging to the gabbro family. The dark sili- 

 cate is a brownish green hornblende of strong pleochroism. 

 It occurs in shreds and irregular masses. Rather more abund- 

 ant is a colorless enstatite of massive habit. The slide gives 

 evidence of dynamic fractures and strains, but is far less broken 

 than the rock of the shear-zone. The somewhat parallel 

 arrangement of the hornblende occasions a very coarse gneissic 

 structure. Under the microscope the enstatite follows a similar 

 foliation. A slide from the north wall of the shear-zone ex- 

 hibits much the same. It is chiefly plagioclase, with a little 

 hornblende, enstatite and a little magnetite. The plagioclase 

 has suffered some dynamic strains and shows faulting of twin 

 lamellae. Figure 2 is a drawing with the camera lucid a. The 

 heavy outlines mark the crystal boundaries as seen without 

 crossed nicols, after which. the twinning has been put in. The 

 cleavages indicate the bisilicates, the rhombic is hornblende, 

 the rectangular enstatite. 



The rock of the shear-zone is dark gray in color and con- 

 trasted with the walls which are lighter. It thus resembles 

 trap. In the hand specimen it is seen at once to be largely 

 made up of garnets and this fact cast the first doubt on its 

 igneous character. It also has the appearance of a rather mas- 



