145 



of the endogenous granite and limestone, are mainly of 

 mechanical origin, may be regarded as highly probable ; and I 

 am inclined to consider that the coarser texture of the older mem- 

 bers of the series is not in general, so far as the mere coarse- 

 ness is concerned, to be regarded as due to their greater age and 

 consequent greater metamorphism and more crystalline charac- 

 ter, but rather as an indication that these were originally the 

 coarser-grained rocks ; the coarse gneisses and schists, at the 

 time of their deposition, probably bearing the relation to the 

 finer-grained, newer rocks — mica slate and argillite — of con- 

 glomerate and sandstone to slate. I realize to how great an ex- 

 tent the last statement clashes with the views held on this subject 

 by many geologists. It is not intended, however, to deny 

 in toto the existence of chemically deposited gneisses. Such 

 gneisses probably exist ; but the facts that have been observed 

 in this region by Mr. Burbank, Prof. Hitchcock, and other 

 geologists, do not favor the conclusion that the gneisses of 

 Eastern Massachusetts are all or principally of this sort. The 

 mechanical origin of the argillite and much of the mica slate is 

 sufficiently obvious, and requires no special proof. In the Har- 

 vard conglomerate, moreover, we have evidence amounting to a 

 demonstration that the argillite was deposited after the manner 

 of more recent rocks ; and since the argillite and mica slate 

 are so conformable with, and pass so insensibly into, the under- 

 lying gneisses, the conclusion is suggested, that these too, though 

 differing more widely than the other rocks named from any 

 modern sediments, have shared the same origin. Fortunately 

 we seem to have better evidence than that derived from analogy. 

 I refer particularly to the narrow belt of altered slate and con- 

 glomerate in Bellingham . This limited band , which was described 

 by Prof. Hitchcock under the head of " metamorphic slate," 

 is of especial interest here as throwing some light upon, or at 

 least suggesting new views concerning, the origin of a portion 

 of our crystalline schists. Its petrological relations to the 

 gneiss will be best shown by a brief sketch of the stratigraphy 

 of that section of the State. 



OCCAS. PAPERS B. S. N. H. — III. 10 



