72 BULLETIN OF THE 



to be intrusive in the Primordiul slates, as well as in the " Huronian " 

 so called, is for this i-eason placed Ijy them at the base of the "Huro- 

 nian." Furthermore, the diabases that are intrusive in all, even the 

 youngest rocks known here, are for that reason considered to be the 

 equivalents of the " Norian," and regarded as older than the " Huro- 

 nian." Now, if the granites are intrusive in the Primordial or Braintree 

 slates, as we know the felsites are intrusive in the granites, and as we 

 know tlie diabases are in all, we naturally should, as we have done, 

 regard these rocks as crystalline eruptive ones, owing their crystalline 

 character to crystallization from the molten magma, and geologically 

 younger than the Primordial, i. e. Palaeozoic rocks. But we see now 

 that we were mistaken. Why not go to the Cordilleras and say of the 

 basalts. These are " Norian " or " Naugus Head"; of the rhyolites. These 

 are "Huronian"; of the trachytes and nevadites. These are " Lauren- 



tiau," or "Formation XX."; of the andesites, 1 Why not station 



one's self by a volcanic crater, and determine, as the eruption takes 

 place, whether Formation V., XV., or XX. is rushing by] 



It seems, then, that in different localities different methods of inter- 

 preting the same facts are resorted to, but for the same purpose and 

 result, — the rocks come out " Huronian " every time. Taking into 

 account the methods pursued ; the hypothesis, yet unproved by any 

 careful, accurate work, that lithological evidence is conclusive ; the 

 assumption that foliation, banding, etc. of necessity prove stratifica- 

 tion ; the practice of inserting faults in the formations wherever it 

 becomes convenient to do so in order to carry out the theory ; that Dr. 

 Hunt, in order to sustain his views, has, in Eastern Massachusetts, 

 directly stated that the granite cuts the felsite, when the reverse is 

 exactly the case, dike after dike of the latter cutting the former, as 

 pointed out by us before,* and since most conclusively shown by Mr. 

 J. S. Diller ; t that the Norian rocks are probably in all cases eruptive 

 basaltic rocks (gabbros) ; that the Keweenawan system has no founda- 

 tion except in erroneous observation, as it conformably overlies the 

 Potsdam sandstone, as we shall .show later ; — taking these and other 

 things into consideration, we feel that the very basis on which the Lau- 

 rentian, Huronian, and other such geological epochs were established, is 

 yet an open question for discussion. Even in Canada the evidence is 

 very far from being clear that the relations of the Laurentian and Huro- 

 nian are what they are supposed to be. The agreement of hundreds 



* Bull. Mus. Comp. Zool., V. 282. 



t Proc. Bost. Soc. Nat. Hist. XX., 354. 



