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strata have been broken by transverse faults producing exten- 

 sive lateral displacement; and in some cases, too, the structure 

 is apparently not synclinal at all, but monoclinal, where strike 

 faults have allowed portions of the slate to drop below the 

 present surface of denudation. I do not consider that the whole 

 thickness of the slate is represented in any part of the Neponset 

 synclinal, but this is to be regarded as a narrow, pinched-up, 

 and broken trough holding only the lower beds of slate. 



The most western exposure of slate is on the New York and 

 New England Railroad, just north of River Street, and near the 

 Hyde Park boundary. It is thin and shaly, with sandy and pebbly 

 layers, and a nearly vertical dip S. 30° E. A few feet farther 

 north-east the beds have apparently been dislocated by a trans- 

 verse fault, and are now exposed in a quarry some ten rods north- 

 west of the railroad. At this point a very clear passage from 

 conglomerate to slate can be made out, and the latter rock is con- 

 siderably contorted ; the general dip, however, remaining the 

 same as before. The next point where the slate appears is on 

 the south-east side of the railroad, a few rods south of Oakland 

 Street. Here we pass from a true slate, plainly ripple-marked 

 and dipping S.E. 70°, through sandstone to conglomerate on 

 the south. It crops again on the same line in Oakland Street, 

 and between that street and Blue Hill Avenue, dipping as 

 before, and with the conglomerate exposed on both sides. The 

 thickness of the slate, which is evidently increasing north- 

 easterly, does not here exceed one hundred feet, supposing it 

 to be doubled upon itself. The passage through sandstone to 

 conglomerate can be observed both ways from the slate. These 

 rocks bear evidence of being squeezed in among the crystalline 

 masses, and the slate especially is frequently much contorted 

 and faulted on a small scale. The nearness of the slates to the 

 crystallines at some points suggests the existence of strike 

 faults concealing part of the conglomerate. The slates of 

 these different localities vary much in color and texture, as 

 this rock is likely to near the conglomerate. 



The area of indigenous petrosilex between the New York and 



