47 



In geology, as in nature generally, hard and fast lines of de- 

 marcation are the rare exception. 



We must rely mainly upon chemical and microscopical 

 analysis for the data necessary for the separation of the pet- 

 rosilex and felsite. But few analyses have been made for 

 Eastern Massachusetts ; so that, although well satisfied of 

 the existence of both these rocks in this region, I have not 

 attempted to trace the distribution of each separately on the 

 maps. There can be no doubt that the petrosilex is much 

 more abundant than the felsite, and hence on the maps, and, 

 frequently, in these pages, I have, for obvious reasons, in- 

 cluded both rocks under the former term. 



Relations of the petrosilex to the Shawmut Group. — 

 But a much greater difficulty now confronts us. The petrosilex 

 of this region is overlaid at many points by a group of rocks, 

 including the well-known petrosilex breccia, which appear to be 

 in every case merely the more or less thoroughly reconsolidated 

 mechanical debris of petrosilex itself. This second group of 

 petrosilicious rocks constitutes one member or division of a 

 formation much newer than the Huronian, for which I have 

 proposed, provisionally (ante, p. 13), the name Shawmut 

 group : a semi-crystalline series which, as will appear in the 

 sequel, underlies the Primordial slate and conglomerate of 

 Eastern Massachusetts, coming between these oldest Paleozoic 

 sediments and the Huronian beds, and appearing to have been 

 formed toward the close of Eozoic time. The petrosilicious 

 portion of the Shawmut group includes rocks of all textures, 

 from a coarse breccia to a compact, homogeneous rock which 

 the naked eye cannot distinguish from the parent petrosilex. 

 They are proved to be of more recent origin than the Huronian 

 petrosilex, not only by their petrological relations, since they 

 everywhere overlie the Huronian, but also and most conclusively 

 by the fact, already stated, that they are composed mainly of the 

 debris of petrosilex, which, where the material is coarse, can 

 be plainly seen to be identical with that which may be re- 

 ferred with certainty to the Huronian system. In short, the 



