PETROGRAPHY 



339 



j magnetite are extremely interesting. They have been described 



and figured by the writer in the Bulletin of the Geol. Soc. of 

 tAmer. — V. 218, and are regarded as due to the reaction of the 



bisilicate or other basic mineral and the feldspar on each other. 



Olivine is found in the gabbros on the lake shore north of Port 

 j Henry, and in the walls of the Split Rock Mine — on Split Rock 



Mountain, Westport. 



In the gneissoid varieties of gabbro, hornblende appears in notable 



preponderance, and in the extreme cases, of very thin lamination it 



is the only dark silicate present. It is quite certainly of secondary 



origin. 



Petrography of the palaeozoic sediments. The microscope 

 yields little in addition to the macroscopic examination. The Pots- 

 dam sandstone shows considerable calcite, in small rhombs mingled 

 with the quartz grains. The Calciferous cherty limestone consists 

 pf fine grained calcite crystals and the chert is devoid of fossil organ- 

 isms of any sort. The others were not ground in sections. 



Dikes. These two townships are not as prolific in dikes as others 

 bo the north. Several have been noted recently by Kemp and 

 Marsters* and determined with the microscope. They are all dia- 

 base or closely related types. There is a bunch of dikes on Mill 

 j)rook, just west of the lake in Port Henry. One or two cut the 

 !>re beds at Mineville. A fine one is in a hill a short distance north- 

 east of Moriah Corners, (No. 56 in 05 of map) where it has been 

 Inined out for an ore body. Several others are exposed along the 

 ! ake shore a mile or two north of Westport, — and others appear in 

 he old iron mines on the west side of the Split Rock ridge. Por- 

 phyries, tho' known in the next township north, have not been met. 



GENERAL CONCLUSIONS 

 The two older series of rocks, the gneisses and the crystalline 

 : I imestones were formed before the intrusion of the anorthosites and 

 il'abbros. These latter came up as great plutonic masses and as 

 jffsetting sheets penetrating the older rocks, and contributing to 

 j heir metamorphism. Much faulting and folding ensued, by which 

 [ he latest intrusions became involved in the earlier rocks in very 



*The trap dikes of the Lake Champlain Valley. Bulletin 107, U. S. Geol. 

 •urvey. 



