26 The Hanging Hills. 



While this theory supposes the several fractures in the series 

 to have been due to the same or similar causes, yet each may have 

 been independent of the others, or only dependent on each other, 

 as one mountain may be dependent upon adjacent mountains. 

 They may have been formed contemporaneously or in succes- 

 sion, following, however, a somewhat definite order and conform- 

 ing to some definite outline. That is to say, East Rock and Mt. 

 Holyoke may have been formed at the same time, or at periods 

 somewhat remote from each other. Or, to confine the illustration 

 to a narrower range, West Peak and Cathole Mountain may have 

 been pushed up through the sandstone at the same time, or one 

 somewhat in advance of the other. But the rents or fissures were 

 practically independent of each other, and so far as appears now, 

 the one might have been without the other. In other words, the 

 hundred or more ridges that Percival represents on his map of the 

 region presupposes a hundred or more rents in the earth's crust, 

 many of them very intimately related no doubt, and still a hundred 

 separate vents. The other theory as to the mode of occurrence of 

 the trap ridges, and one which was presented by Prof. W. M. Davis, 

 of Harvard University, in a somewhat elaborate paper at the meet- 

 ing of the American Association for the Advancement of Science at 

 Buffalo, August, 1886, may be briefly stated in the author's own 

 words, as follows — "that nearly all these ridges are the outcropping 

 edges of contemporaneous lava overflows, and that the outcrop of a 

 single sheet is repeated several times by faults nearly paralled to the 

 strike of the beds." 



A detailed statement of the evidence bearing on the subject is 

 promised in the forthcoming Seventh Annual Report of the U. S. 

 Geological Survey, for which we shall look with interest. But as 

 we understand the theory, it may be briefly stated thus. The 

 overflow of the trap, whether from few or many vents, formed a 

 continuous and essentially horizontal sheet over a large portion of 

 the area now included in the trap region, before the occurrence of 

 the ruptures and dislocations that produced the striking features 

 so apparent now : while the separate ridges, commonly supposed 

 to be due to separate eruptions, are the result of profound fractures 

 and faulting of the whole series of rocks, including not only the 

 sandstone and the trap, but the crystalline rocks that lie at still 



recent traps, or between that still buried in the sandstone and that exposed upon 

 the surface. Tliis distinction is made by Prof. W. N. Rice in a recent article — 

 Am. Jour, of Science, December, 1886 — and illustrated by reference to some ex- 

 posures in the vicinity of Tariffville, Conn, 



