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phism, a common source of the coarse schistose gneisses. And 

 since the Montalban seas, in common with the seas of all ages, 

 must have formed — and, as the existing remnants testify, act- 

 ually did form — conglomerates ; and in view of the facts 

 already cited concerning the alteration of this class of rocks, 

 what conclusion is more natural ? 



The chemical gradation . shown to exist in the rocks of the 

 Huronian system is repeated in our Montalban strata, the 

 proportion of the alkalies potash and soda steadily diminish- 

 ing as we proceed from the granite and gneiss, at the base of 

 the series, to the argillite at the top ; and there is very evi- 

 dently, as pointed out by Dr. Hunt, a concomitant decrease 

 in the capacity for metamorphism which the various rocks 

 possess. 



In this and the succeeding paragraph I have stated briefly 

 the main points in the evidence which the stratigraphy of this 

 region affords that the rocks here designated as Montalban are 

 newer than the Huronian. A glance at the map will show 

 that the geographic disposition is such that, in going from the 

 older Montalban rocks to the newer, from the gneiss to the 

 argillite, the passage is away from the Huronian. In other 

 words, the only Montalban strata in contact with the Huronian 

 are, generally speaking, those forming the base of this 

 system, — the granite and granitoid gneiss ; and at most points, 

 viz., everywhere between Salisbury and Natick, we find 

 this Montalban base in contact with the rocks that we 

 have learned to recognize as the summit of the Huronian. 

 This is certainly a strong indication of the more recent age of 

 the Montalban, and it becomes stronger when we observe fur- 

 ther, that from Salisbury to Westborough the gneisses dip away 

 from the Huronian diorites, appearing to overlie them. This 

 point is offset, however, by the contrary dips which obtain fur- 

 ther south and in the Buzzard's Bay region. Still it is now 

 generally recognized by geologists that rocks may, and often 

 do, dip toward older formations. The Triassic sandstones of 

 this State are a good example among newer rocks. In the 



