180 



especially since Dr. Hunt l has regarded them as identical in 

 age with the rocks in the vicinity of Boston forming the basic 

 division of the Shawmut group, though referring them, along 

 with the latter, to the Huronian system. Without considering 

 their relations to the rocks of this vicinity, it is sufficiently evi- 

 dent to my mind that the altered slates of Newport cannot be 

 Huronian, since they appear to be cut by, and at least overlie, 

 granite which is almost certainly Montalban. In part these 

 slates are brecciated, and hold jaspery patches and pebbles of 

 petrosilex ; other small portions exhibit a tendency to develop 

 amygdaloidal characters ; while the mass of the rock is very 

 similar lithologically and petrologically to the slate forming the 

 island in the Blue Hill granite in Milton. 



I have never obtained any reliable data on which to base 

 even a probable estimate of the thickness of the Shawmut 

 group. Such indications as we have, however, point to the 

 conclusion that, comparatively speaking, the volume is not 

 enormous ; possibly two thousand feet as a maximum. The 

 little that is known of the stratigraphy of this group will be 

 most conveniently introduced in connection with the structure 

 of the succeeding formation, — the Primordial. 



^roc. Bost. Soc. Nat. Hist., xiv., 46. 



