OUTLINE OF THIS PAPER. 



.Mount Greylock, or Saddle mountain, in northwestern Massachusetts, has been 

 studied off and on by geologists for seventy years. The literature is given on p. 131. 

 The general synclinal structure of the mountain is well known. This description is 

 based upon the new topographic map of the l r . S. Geological Survey, and upon 

 the results of receni orographic science. Mr. .1. Eliot Wolff lias done the petrographic 

 work. 



The mountain consists mainly of one central and two lateral subordinate ridges, 

 all trending about north northeast to south-southwest. With its spurs it forms a 

 topographic unit and measures lGi miles in length and averages about .'">[ in width. 

 Its aspects from the north, south, east, and west are described on p. 184 (Pis. xn, 

 xiii-xv). The •• saddle" is formed by a depression in the southwesterly bend of the 

 central ridge, between Greylock summit (3,505 feet) on the north and Saddle Ball 

 (3,300 feet) on the south. These are about 2 miles apart, and the lowest part of the 

 saddle is 00.5 feet lower than Greylock summit. 



Structural. — -The rocks are all metamorphic and of few kinds, crystalline lime- 

 stone, quartzite, and schists. The key to the structure is in the distinction between 

 cleavage foliation and stratification foliation. The principal recent and older liter- 

 ature of that subject is given on p. 137. The phenomena of cleavage and stratifica- 

 tion and pitch, as they occur on Greylock, are illustrated by ten typical cases. These 

 lead to the adoption of the following structural principles: I. Lamination in the 

 schist or the limestone tnaj lie either stratification foliation or cleavage foliation or 

 both, or sometimes, in limestone at least, '• false bedding." To establish conformability, 

 the couformability of the stratification foliation must be shown. II. Stratification 

 foliation is indicated by: («) the course of minute but visible plications; (/*) the course 

 of the microscopic plications; (<•) the general course of the quart/, lamina' whenever 

 they can be clearly distinguished from those which lie in the cleavage planes. III. 

 Cleavage foliation may consist of: (a) planes produced by or coincident with the 

 faulted limbs of the minute plications; (6) planes of fracture, resembling joints on a 

 very minute scale, with or without faulting of the plications; (c) a cleavage approach- 

 ing slaty cleavage, in which the axes of all the particles have assumed either the 

 direction of the cleavage or one forming a very acute angle to it. and where stratifica- 



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