58 NEW YORK STATE MUSEUM 



Grenville strata generally having been removed or at least much 

 worn down parallel to the foliation. Two large areas — one across 

 the northern side of the quadrangle, and the other south of Blue 

 ^fountain and Eagle lakes — afford fine illustrations of such 

 phenomena. In those mixed gneiss masses where the Grenville is 

 unusually resistant and intimately involved with the granite or 

 syenite, the rocks may stand out in rather bold relief as, for instance, 

 south and southwest of Unknown pond, and west of the south 

 end of Long lake. 



It is important to note that the main axis of elevation of the 

 Adirondack region which extends across the Blue ^Mountain quad- 

 rangle is cut through by two of the few lowest passes with maxi- 

 mum altitudes of about 1800 feet. One of these is the Grenville 

 valley across the quadrangle from Pine lake westward to Blue 

 ^fountain lake, and the other is the broad lowland belt of mixed 

 gneisses across the northern side of the quadrangle. 



The highest mountain masses always consist of facies of the 

 great syenite-granite intrusive body with rarely more than slight 

 amounts of admixed Grenville. Such rocks are exceedingly resist- 

 ant to weathering and erosion, and it would require a \ery long 

 time (a few million years perhaps) to cut down these mountains 

 to the general level of the valleys now occupied by the Grenville 

 strata. 



The gabbro appears to be about as resistant as the syenite or 

 granite, but its small masses do not permit any notable topographic 

 influence. Where completely surrounded by weak Grenville, the 

 gabbro usually stands out distinctly as small knobs like those either 

 side of Spragi,ie pond. The rather prominent gabbro knob between 

 3 and 4 miles east of Long Lake village shows outcrops of inclosing 

 rock on one side only, and it is possible that more or less Grenville 

 or mixed gneiss has been removed from around it. 



Influence of Rock Structures 

 AA'here relatively weak Grenville strata dip downward against 

 large masses of homogeneous syenite or granite, high and very steep 

 slopes, simulating fault scarps, are usually developed as a result 

 of weathering and erosion. Cases in point are the steep escarpment 

 on the western face of the mountain from i to 2 miles south of 

 Rock lake, and the steep eastern front of Waterbarrel mountain, 

 each rising fully 700 feet. There is some reason to believe, as 

 alreadv 5U2^2:ested. that a mass of Grenville has been removed from 



