MOUNT GKEYLOCK. 127 



deepens at two points. In the eastern syncline, P, the deeper part of the northern 

 depression is shown to be about under the center of Ragged mountain, while in the 

 central one, Q, the deeper part of the northern depression seems to be about 2 miles 

 farther south, between Greylock and Saddle Ball and near Greylock summit. The 

 northern side < >r edge of this great double trough is at the extreme north end of the Grey- 

 lock mass ; section Q begins at Clarksburg mountain, and its southern edge is between 

 7 A and 8£ miles distant, near Round Rocks and on the southeast spur of Saddle Ball. 

 South of these main troughs is another pair, the centers of which lie west of Cheshire 

 reservoir. To the west of these two long axes the mountain mass is made up of 

 numerous minor folds, which do not show the continuity seen in P and Q. It will be 

 seen that the direction of these two main synclincs represented by P and Q is north- 

 northeast by south-southwest, thus nearly parallel with the direction of the valley 

 lying between the Clarksburg granitoid gneiss mass and Hoosac mountain, and that 

 at the south end they converge and perhaps unite in the harrow schist ridge 

 between Berkshire and Lanesboro villages. Traversing the folds of this canoe-like 

 complex synclinorium is a cleavage foliation, sometimes microscopically minute, dipping 

 almost uniformly east. This cleavage foliation is distinct from the "slaty cleavage," 

 early described by Sedgwick, Sharpe, and Sorby and reproduced experimentally by 

 Tyndall and Jaunettaz, and consists sometimes of a minute, abrupt, joint-like fractur- 

 ing of the stratification lamina', but more usually of a faulting of these laminae as the 

 result of their extreme plication — a mode of cleavage ("Ausweichuugsclivage") so 

 well described by Heim and recently reproduced in part by Cadell by a slight 

 modification of the experiments made by Prof. Alphouse Favre, of Geneva, in 1878. 

 (See foot-notes, p. 137.) This slip cleavage, when carried to its extreme, results in a 

 form of cleavage very much approaching, although not identical with, slaty cleavage. 

 To the unaided eye all traces of stratification are lost, and even under the microscope 

 they are so nearly lost as to be of no avail in determining the dip. This and the 

 regular slip cleavage often occur in close proximity. 



Lithologic stratigraphy. — There are five more or less distinct horizons in the Grey- 

 lock mass. The following descriptions are based upon Mr. Wolff's petrographic deter- 

 minations, beginning above: 



The Greylock .schist (Sg). Muscovite (sericite), chlorite, and quartz schist, with or 

 without biotite, albite, magnetite, tabular crystals of interleaved ilmenite and chlorite, 

 ottrelite, microscopic rutile, and tourmaline. Thickness, 1,500 to 2,200 feet. Part of 

 Emmons's pre-Cambrian or Lower Taconic No. 3 (" talcose slate"), Walcott's Hudson 

 River (Lower Silurian). 



Bellowspipe limestont (Sbp). Limestone more or less crystalline, generally mica- 

 ceous or pyritiferous, passing into a calcareous schist or a feldspathic quartzite, or a 

 fine-grained gneiss with zircon and microcline, in places a noncalcareous schist. The 

 more common minerals are graphite, pyrite, albite, microscopic rutile, and tourma- 

 line; rarely, galena and zinc blende. Thickness, 000 to 700 feet. Part of Emmons's 



