EASTERN MASSACHUSETTS. 



435 



coarsely crystalline aspect and tlie great disturbance and almost complete 

 absence of stratification whicL. it everywhere exhibits, but also by the general 

 fact that it cuts^ as an exotic, all the other members of the Huronian system. 

 In fact it pierces, in its well-nigh universal extravasation, every rock iu this 

 region, save the Naugiis Head series and the newer uncrystallines. In its 

 geographical distribution we have a strong indication that the granite belongs 

 to the Huronian system ; for it is co-extensive with that system, and does not 

 occur beyond its limits. And it will be shown farther on that its lithological 

 relations point indubitably to the same conclusion. But its petrology makes 

 it clear that, if the granite is referred to the Huronian series, it nuist be re- 

 garded as the lowest, and hence the oldest rnen^ber of that series. It appears, 



in fact, to be the foundation of the Huronian system in Massachusetts 



Although well satislied that a large proportion of the granite has been in a 

 state of igneous plasticity, yet its relations to the stratified petrosilex and the 

 many traces of bedding which it still retains forbid me to believe that the mass 

 of this rock has been elevated from any vast depth ; it seems rather like an 

 extensive stratified formation which has been softened m sitUf and then to a 

 greater or less extent forced out of its normal position by the pressure of sur- 

 rounding and overlying terranes " 



Mr. Crosby now classed nearly all of the felsito under that obsolete 

 term petrosilex, following in this, as in almost all of his work, the ideas 



of Dr. Hunt. Of the felsite and its relations to other rocks he says 

 (I. c, pp. 47-G9) : 



'* The petrosilex of this region is overlaid at many points by a group of rocks, 

 including the well-known 2)etrosilex breccia, which appear to be in every case 

 merely the more or less thoroughly reconsolidated mechanical debris of petro- 

 silex itself. This second group of petrosilicious rocks constitutes one member or 

 divisii)n of a formation much newer than the Huronian, for which I have pro- 

 posed, provisionally, the name Shawmut group ; a semi-crystalline series 

 which, as will appear hi the sequel, underlies the primordial slate and con- 

 glomerate 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 Shawnuit group 

 includes rocks of all textures, from a coarse breccia to a compact, homogeneous 

 rock whicli the naked eye cannot distinguish from the parent petrosilex. 

 They are })roved to be of more recent origin than the Huronian petrosilex, not 

 only l)y their petrohigical relations, since they everywhere overlie the Huro- 

 nian, bat also and most conclusively by the fact, already stated, that they are 

 composed mainly of the debris of ju-trosilex, wliicli, where the matenal is 

 coarse, can be plainly seen to be i'lentical with that which may be referred 



with certainty to the Huronian system That the Huronian petrosilex 



is now for the most part a stratified rock, and was originally wholly so, I can- 

 not doubt; and it appears most probable that the conditions presiding over its 



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