PETROGRAPHY 339 



magnetite are extremely interesting. They have been described 

 and tignred by the writer in the Bulletin of the Geol. Sue. of 

 Amer. — 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 

 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 cmite 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 

 of 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 

 to 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 

 brook, just west of the lake in Port Henry. One or two cut the 

 ore beds at Mineville. A fine one is in a hill a short distance north- 

 .east of Moriah Corners, (TsTo. 56 in 05 of map) where it has been 

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

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

 the 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 

 limestones were formed before the intrusion of the anorthosites and 

 gabbros. These latter came up as great plntonic masses and as 

 offsetting sheets penetrating the older rocks, and contributing to 

 their metamorphism. Much faulting and folding ensued, by which 

 the latest intrusions became involved in the earlier rocks in very 



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

 Survey. 



