LOWER SILURIAN. 



213 



The following section, of a region about two miles in length, represents the rocks of 

 Monument Mountain, situated half way between Stockbridge and Great Barrington, in 

 Berkshire Co., Massachusetts. Below, to the west (left, near the Housatonic river), the 



395 A. 



§i si n H n s 1 q 1 



Section across Housatonic Valley and Monument Mountain. 



Stockbridge limestone (Chazy or Trenton), is seen bent into a low fold. Over it, there 

 is a bed of mica schist and gneiss (once a bed of sediment), bent in the same manner; 

 then, to the east of this, there is a great stratum of quartzyte (once a bed of quartz 

 sand), 200 to 250 feet thick, dipping southeastward; this quartzyte, going eastward, 

 is overlaid by a second stratum of gneiss and mica schist, 300 to 400 feet thick, and 

 by a second of quartzyte, 200 feet, or so, thick. 



The limestone, to the west of the section, dips under a ridge of mica slate, called 

 Tom Ball, and comes up on the west side in nearly vertical beds. Tom Ball, a ridge 

 800 or 900 feet high above the Housatonic river, is a portion of the overlying slate, 

 that was squeezed up in a deep downward flexure, or synclinal, of the underlying 

 limestone; and to this the slate constituting it owes its existence in a ridge; for, where 

 the limestone raised its back in anticlinal folds, the rocks above were broken from top 

 to bottom, and so became an easy prey to denuding agencies. The Tom Ball synclinal 

 is a shallow one at the north end (V 4 , Fig. 395 B), where the limestone may be seen 

 (near W), dipping under the slate; but, at the south end, a very steep and deep one, 

 with the axial plane inclined eastward (V 4 , Fig. 395 C), both the slates and the lime- 

 stone beds east and west of them having a high easterly dip. The limestone region of 



Fig. 395 B. 



V-2 A2 Vi Ai G 



Section from Glendale westward, through north end of Tom Ball. (G, Glendale ; V 4 , Tom Ball.) 



Fig;. 395 C. 



W 



H 



Alluvial plain S. of Mon. Mt. 



Section across Long Pond Valley through Southern half of Tom Ball. 



the Green Mountains is full of examples of such folds. The eastern (right) part of the 

 section in Fig. 395 B exhibits their character directly north of Monument Mountain, 

 where there are two narrow synclinals (V, V' 2 ), with anticlinals of limestone (A 2 , A 1 ), 

 m place of the gently inclined strata of the mountain; and the synclinals are so narrow 

 that only a small part of the overlying schist is pinched in, and no quartzyte. The 

 culminating range of the Green Mountains, south of Vermont, stands along the 

 western boundary- of Massachusetts, and is called the Taconic range. Mount Wash- 



