1884.] 21 [Crosby. 



are not in the part of the felsite nearest the conglomerate, nor is 

 there any indication of a gradual passage between the con- 

 glomerate and felsite. Furthermore, the segregated veins of 

 quartz, to which Dr. Wadsworth rightly refers as evidence of 

 thermal action, are in the centre of the pinite masses and 

 do not touch the flinty felsite. Hence they are evidence 

 that the pinite is the most highly altered portion of the rock, 

 resulting from the hydration of the felsite by meteoric waters 

 which have followed the course of the quartz veins and 

 decomposed the felsite on either side. The purest and greenest 

 pinite is next to the quartz, and from this there is a gradual pas- 

 sage within a few inches to the unaltered purple felsite. These 

 small veins of quartz are common in both the conglomerate and 

 felsite of this region. The composition of both the pinite and 

 felsite, as shown by chemical analyses, is favorable to the view 

 that the former has resulted from the alteration of the latter, 

 while it is difficult to see how hydrothermal action could give 

 the pinite or the pinite conglomerate the composition of the fel- 

 site. One of the strongest arguments against Dr. Wadsworth's 

 view is the undeniable fact that the conglomerate is composed of 

 the debris of this same purple felsite and green pinite, and the 

 conglomerate has this peculiar composition only in the immedi- 

 ate vicinity (within a few rods) of the felsite. The source of the 

 pebbles is plain ; and I think we have here entirely satisfactory 

 evidence that the felsite is older than the conglomerate. The 

 relations of the two formations would scarcely be clearer if this 

 were a ledge of granite surrounded by a conglomerate composed 

 of pebbles of the same granite. And there is not a shadow of 

 proof that the pinite debris has been produced in the conglom- 

 erate. Pebbles of this mineral are common in the conglomerate 

 in all parts of the Boston basin, and it is always clearly an im- 

 ported constituent. Dr. Wadsworth apparently set out to con- 

 test the view that the Boston conglomerates are younger than the 

 felsites of this vicinity; an undertaking that would seem to have 

 been hopeless from the beginning, in view of the facts that every 

 variety of felsite occurring in this region is represented in the 

 pebbles of the conglomerate, and that the felsite pebbles are 

 abundant in all parts of the conglomerate, and not alone in the 

 Milton conglomerate. 



