254 



Obscure outcrops of homogeneous gray slate are numerous 

 between Belmont and Orchard Streets, near Lexington Street, 

 but they afford no reliable indication of the dip. The slate on 

 North Street, just west of Common Street, is gray, distinctly 

 stratified, and without cleavage ; it dips N. 20°-25° W. about 

 60°. One-half mile east, a short distance west of School 

 Street and midway between Belmont and Washington Streets, 

 a soft, slippery, well-cleaved slate has substantially the same 

 dip as the last. The foregoing are all the exposures of the 

 uncrystallines with which I am acquainted in Watertown ; and 

 while, in connection with the drift, they show that the rock is 

 mainly slate, the structure of the region remains almost entirely 

 a matter of conjecture. These meagre data, however, afford 

 some ground for the opinion that the rocks of this disti'ict are 

 thrown into a series of folds running parallel with the crystal- 

 line border on the north, the position of one anticlinal axis 

 being marked by the conglomerate. 



In Belmont and Arlington the uncrystallines do not reach the 

 surface, and the same is nearly true for Cambridge. Excava- 

 tions made for a sewer in the yard at Harvard College, in 

 1871, exposed the slate at a depth of twelve feet, with a gentle 

 southerly dip. The general absence of conglomerate from the 

 drift of this large area of thirty or thirty-five square miles, 

 lying between the conglomerate of Brighton and Newton on 

 tlie south (this border being pretty accurately marked by the 

 Boston and Albany Railroad), and the crystallines of Maiden, 

 Medford, Arlington, Belmont, and Waltham on the north, to- 

 gether with the testimony of such outcrops as occur, renders it 

 certain that the underlying rock is almost wholly slate. In 

 Somerville, as is well known, there are abundant exposures of 

 this variety. This city and, probably, Cambridge constitute a 

 region of monoclinal dips. Except at a few points, where the 

 slates are locally disturbed by intrusives, the strike in Somer- 

 ville ranges between E.-W. and N. 60° W., averaging about 

 W.N.W. ; while the dip, with rare exceptions, is southerly, 

 and usually at low angles, the average inclination probably not 



