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NEW YORK STATE MUSEUM 



over. Frequently the limestone becomes more magnesian and is 

 full of the altered pyroxene crystals, which give it the well-known 

 green, serpentinous mottlings. This ophicalcite or ophiolite is 

 widely familiar under the name " Moriah marble," by which trade 

 designation it has been placed on the market as an ornamental stone. 

 G. P. Merrill has shown that the serpentine has resulted from the 

 alteration of a white pyroxene.* The limestones have undoubtedly 

 been penetrated by igneous dikes before their metamorphism, for these 

 show in broken fragments with the limestone in between. Whether 

 all the interbedded, black schist was of this character is doubtful. 



The limestone series rests on the gneiss and is later in age. 



General characters of the rocks of the gabbro family. The 

 gabbros are unquestionably true, igneous rocks of invariable plu- 

 tonic habit when not rendered more or less gneissic by mountain- 

 making disturbances. Passages of the one into the other can be 

 traced. But even in the purely plutonic or granitoid structures the 

 microscope shows widespread crushings and strainings, the results of 

 dynamic disturbances. The typical anorthosites exhibit nearly pure 

 f eld spathic aggregat es, specially in the interiors of the Norian 

 mountain ridges. The darker, basic gabbros appear on the skirts of 

 the latter or as more remote outliers. They consist of plagioclase, 

 green monoclinic pyroxene, hypersthene, brown hornblende, titani- 

 ferous magnetite, almost invariable garnet and alteration products 

 from all these. Olivine has been discovered but does not appear to 

 be as abundant as in the analogous Canadian exposures described by 

 F. D. Adams. 



General characters of the Cambro-Silurian sediments. The 



earliest of the sediments is the Potsdam sandstone. It rests on all 

 the members of the Laurentian in one place and another and sets 

 up into them as embayments. It reaches a maximum altitude of 

 500 feet above tide at the first Y of the Mineville railway, out from 

 Port Henry. None other of the sediments rises as high at this. 

 Along the lake shore, the Potsdam is succeeded by the Calciferous 

 Limestone, which appears just north of Port Henry and in Westport. 

 In the latter town we find also the still higher Chazy and Trenton. 

 Less attention was given to these than to the crystalline rocks, 

 although later on some further details and lists of fossils are appended. 



* Proc. U. S. Natl Mus. XII, 595. 1890. 



