56 ■ NEW YORK STATE MUSEUM 



west in the Johns Brook valley. The Johns Brook valley heads on 

 the east side of the summit of Mount Marcy and when the line of 

 the depression is followed to the southwest across the divide, it 

 passes straightaway down the steep-sided depression of Skylight 

 brook. The line is almost mathematically parallel with the great 

 fault-valley of the Ausable lakes on the southeast and the fault- 

 valley of the Cascade lakes on the northwest. The steep-sided 

 depression which contains Avalanche lake, Lake Colden and the 

 Flowed lands runs also parallel to it but is less extended. The same 

 is true of the precipitous pass northwest of Pitchoff mountain. In 

 the extreme southeastern corner of the quadrangle the valley of 

 Niagara brook with the wonderful and precipitous escarpment of 

 Niagara mountain, serves to further emphasize these great northeast 

 and southwest lines of faulting, which are the chief structural 

 features of the quadrangle. 



The lines of faulting are not always single. In the summer of 

 191 5 H. L. Ailing was able to demonstrate the double character of 

 the northeast fault below the Lower Ausable lake. Study of the 

 map will show two valleys, separated by a narrow ridge and each 

 occupied by a brook. Two parallel faults are responsible for the 

 depressions and one has produced a " rotten stone quarry " for 

 macadam at the foot of the lake. At another point much secondary 

 calcite has developed in the sheared and brecciated rock. 



The crushing of the country rock along the faults and the subse- 

 quent staining of the feldspars red or reddish brown by infiltrated 

 iron salts, sometimes give the strongest impression to the observer 

 that a red granite intrusive mass is before him, and one that is dif- 

 ferent from the wall-rocks. In a number of instances the writer 

 has been puzzled by these appearances but has in the end concluded 

 that they were secondary. 



In smaller but still impressive gulches minor fractures are also 

 brought out. One which is a favorite and easily accessible by walking 

 from the Keene valley, is found in the northeast prolongation of the 

 depression between Roosters Comb and Snow mountain. The gulch 

 is so small as not to be shown on the map. It is known as Wash- 

 burn flume, and is a narrow, precipitously walled trench, with sides 

 as true as masonry and i^ miles long. 



On the road which runs southeast from the Keene valley along 

 the course of Beede brook to Chapel pond (Chapel pond road), and 

 at the first strong rise above the valley, there is a rock cutting in 

 greatly decomposed anorthosite, which happens at this locality to 



