48 



so-called porphyries of this region are separable chronologic- 

 ally into two kinds, which are genetically related, and the 

 real petrosilex, which belongs exclusively to the Huronian 

 formation, is the oldest. The non-petrosilicious members 

 of the Shawmut group are in the main lithologically dis- 

 tinct from anything in the Huronian system ; and the same 

 is true of the petrosilicious portions of this group, where 

 the breccia or conglomerate character is well developed. But 

 where, as sometimes occurs, the recomposed rock has been 

 formed as a fine-grained, compact, flinty-looking sandstone or 

 slate, or, as is also perhaps not infrequently the case, has 

 reached this condition by the loss of the breccia structure 

 through subsequent metamorphic processes, the separation of 

 the Shawmut rock from the Huronian petrosilex becomes a 

 matter of extreme difiiculty. The parent and offspring are 

 then chemically and, to the naked eye, lithologically indistin- 

 guishable, and we are obliged to rely upon their petrological 

 relations and microscopical characters for the means of dis- 

 guishing them. And since, throughout its distribution, the 

 new, pseudo petrosilex lies for the most part directly upon the 

 old, sometimes in such a manner that the finer-grained, more 

 truly petrosilicious portions are brought in contact with the 

 vastly more ancient parent rock, from which the whole were 

 derived, and which they so closely resemble; and since there 

 is not wanting abundant evidence that all these rocks have 

 .been subject in a great degree to the action of disturbing and 

 metamorphic agents subsequently to the deposition of the 

 Shawmut group, not only increasing their strong lithological 

 resemblance, but adding immensely to the complexity of their 

 petrological relations, — it will be readily seen how puzzling a 

 problem it is to determine, for the purpose of accurate delinea- 

 tion on the map, or for detailed study, the limits of the Huro- 

 nian petrosilex. So intimate is the association of the Huronian 

 petrosilex and the petrosilicious rocks of the Shawmut group, 

 and so complete, apparently, is the gradation existing be- 

 tween the typical varieties of the former rock and the normal 



