GEOLOGY OF THE LAKE PLACID REGION 



61 



some of the gneisses are the oldest rocks present. They repre- 

 sent the remnants of a once extended series that formed all the 

 country. They have been invaded and broken np into small de- 

 tached areas by the igneous anorthosites. The intrusions took 

 place at quite profound depths in the earth, because the anortho- 

 sites have all the characteristics of rocks that have cooled and 

 crystallized under pressure and slowly. The limestones were 

 much affected by the neighboring masses of igneous rock and may 

 owe to their influence the great numbers of included pyroxenes 

 and other silicates. 



Many facts otherwise inexplicable are accounted for by this 

 conception, as for instance the piresenoe opposite Gascadeville of a 

 small mass of limestone, a sedimentary rock, in a great mountain 

 oif anorthosite, an igneous one. The limestone on the north- 

 western extremity of Pitchoff is a still more striking case, while 

 fragments of quartzite have been found in the anorthosites of the 

 high peaks, as for instance on the summit of the Gothics. The 

 exact relations of the granites to the anorthosites in time, are 

 uncertain, but the granites are probably later. 



After the intrusion of the anorthosites great metamorphism 

 ensued, that crushed the component minerals and produced much 

 gneissoid foliation. The rocks were apparently under such com- 

 pression and strain, that they flowed like a viscous fluid, and the 

 minerals became strung out in linear arrangement. It all 

 occurred however before the deposition of the Potsdam sand- 

 stone, because we find the latter to the north and east resting 

 unchanged on the older metamorphic rocks. 



The trap dikes were certainly intruded after the metamor- 

 phism, for they show no evidence of having been squeezed or 

 sheared. In the region to the north, H. P. Gushing has found 

 dikes that cut the old crystallines, but stop at the Pots- 

 dam, and do not penetrate it. He therefore has concluded that 

 they were intruded before the Potsdam was deposited. The 

 writer has found others in the Lake Ghamplain region that 

 pierce strata even as late as the Utica slate. Glearly therefore 

 two series are present in the mountains, but to which of the two 



