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degrees, we have between the Mystic and the Fitchburg Rail- 

 road — nearly one and a half miles across the strike — an appar- 

 ent thickness of not less than thirty-five hundred feet, and, if 

 the Cambridge slate is included, very much greater. This fact 

 alone is sufficient to render the existence of strike faults with 

 the downthrow on the north exceedingly probable ; but other 

 general considerations point to the same conclusion, though 

 the slates themselves are everywhere too homogeneous to show 

 faulting through the repetition of particular strata. The 

 principal elevations of Somerville form two well-marked, though 

 discontinuous, ranges, which are nearly parallel with the strike 

 of the slates. The more southerly of these includes Spring, 

 Central, and Prospect Hills and the elevation occupied by the 

 McLean Insane Asylum ; while the northern range is composed 

 of College and Winter Hills and Mt. Benedict, and appears to 

 be continued by Bunker and Breed's Hills in Charlestown. 

 Between these two is a much lower and less distinctly marked 

 range, which, including the principal part of East Somerville, 

 is recognizable at intervals to the western border of the city. 

 The northern slopes of the two main range's are sensibly the 

 steepest, and in all respects the topography of the region is 

 decidedly favorable to the fracture theory of its origin. The 

 bases of the declivities, too, are marked at most points by 

 extensive dykes parallel with their trends. On the north side 

 of the more southern of the two long dykes in the north part 

 of Somerville the slates show usually a gentle northerly dip, 

 as if from the lifting power of the dyke ; but toward the other 

 dyke this changes rapidly, almost abruptly, to a high, and in 

 some cases vertical, dip to the south. 



The escarpment formed by the last-mentioned dyke, toward 

 the east, is very suggestive of a fault. About two hundred 

 feet north of, and parallel with, this escarpment is a second, 

 less distinctly marked and about half as high, which shows no 

 dyke, but indications of one, and slate dipping S.S.W. 

 40°-80°. The low island in the Mystic River crossed by the 

 Eastern Railroad is on this line, and shows slate dipping south. 



