144 NE W YORK STATE MUSEUM 



opposite. In the bottom of the rift and also at the eastern base of 

 the Chimney rock mass there are great accumulations of angular 

 blocks of rock, often 10 to 20 feet across, which have rolled down 

 the steep slopes since the development of the chasm. 



The fact that, not many years ago, a forest -fire made an almost 

 clean sweep of the vegetation from this portion of the mountain 

 causes the chasm and its immediately surrounding rock masses to 

 be very plainly visible. 



THE ROCKS 



All the exposed rocks are of Precambric age, the main mass of 

 the mountain consisting of fairly homogeneous, moderately gneiss- 

 oid, granitic syenite, with Grenville strata resting against the western 

 side. The rift is wholly developed within the Grenville strata, which 

 there have a visible thickness of about 250 feet. These rocks, which 

 are very distinctly stratified in layers from 6 inches to 4 or 5 feet 

 thick, are rusty looking biotite-quartz-feldspar gneisses, greenish- 

 gray pyroxene (coccolite) -feldspar gneisses, and some beds of 

 quartzite. 



On the western side of the chasm the rocks strike N 20 E or 

 parallel to the rift and dip 50 degrees westward, this strike and dip 

 being uniform down the whole western face of the mountain. On 

 the eastern side of the chasm the strike and dip are quite different, 

 being N 40 W with greatest dip (at Chimney rock) of 20 degrees 

 toward the northeast. 



It is important to note that in spite of such marked differences in 

 strike and dip, the rocks on opposite sides of the rift are of exactly 

 the same character in every respect, and it is certain that they were 

 once parts of a continuous mass. 



Prominent joint-planes, mostly approximately at right angles to 

 the bedding planes, are common, so that frequently large joint- 

 blocks loosen and slide down the steep slopes. 



The exact character of the Grenville strata immediately beneath 

 the exposed gneisses just described is not known though, as will 

 be explained below, they are quite certainly relatively weaker rocks. 

 That they are probably either limestones or at least limestone (or 

 calcareous) strata interbedded with gneisses is strongly suggested 

 by the fact that Grenville beds very similar to those of Chimney 

 mountain arc often directly associated with limestone in the central 

 and southern A.dirondacks, such association having been frequently 

 observed by the writer in the vicinity of Indian Lake village and in 

 the valleys between that village and Chimney mountain. At the 



