the Structural Relations of the Huronian. 231 



there appears below these, division 2, from the base upward, 

 of the Original Huronian, the greenish chloritic and epidotic 

 slate group, which has been shown by Irving to be a surface 

 volcanic* Below this is found formation 1, the gray quart- 

 .zite which grades down into the conglomerate just described. 



This locality then gives decisive evidence of an erosive 

 unconformity between the lowest member of the Original 

 Huronian and the Basement Complex. The intricate history 

 written in the contorted gneisses and schists, in their intrusions 

 by granites, in the subsequent pegmatization of both, is evi- 

 dence that a great length of time was taken for the genesis of 

 the basement complex. Then a deep truncation must have 

 occurred before these granites could be found as surface rocks. 

 Finally an orographic movement depressed the crystalline 

 complex below the sea and the basal conglomerate and gray 

 quartzite are the opening chapter of the Huronian. It is then 

 manifest that the time gap represented by the unconformity 

 between the Basement Complex and Huronian is of the first 

 magnitude. 



General. — As a result of our observations it is evident that 

 at two distant points, one near the west end of the Original 

 Huronian area and the other near the east end, are great phys- 

 ical breaks between members of the lower division of the 

 Huronian and a more ancient crystalline complex which was 

 designated by Logan and Murray as Laurentian. The nature 

 of these breaks is such as to make it impossible that they can 

 have been local, and the conclusion therefore appears war- 

 ranted that in the typical district mapped in detail by Logan 

 and Murray, between the Huronian and the Basement Complex 

 there was an immense period of time. As further evidence of 

 this break is the very different lithological characters which 

 the Huronian and basal complex have. In the latter are 

 known no rocks which have been demonstrated to be of sedi- 

 mentary origin or even surface volcanics, while many of them 

 are plainly deep-seated igneous rocks. The major part of the 

 Huronian rocks, on the other hand, are so little altered that 

 their fragmental character is generally seen in the field, and is 

 always evident at a glance when the rocks are examined in 

 thin section. Finally, the igneous rocks associated with these 

 detritals are in large part surface volcanics. 



If the foregoing conclusions are correct, it follows that the 

 pre Keweenawan rocks of the north shore of Lake Huron are 

 separable by unconformities into three series ; a Basement 

 Complex, a Lower Huronian and an Upper Huronian. These 



* Ib there a Huronian Group? R. D. Irving. This Journal, III, 1887, vol. sxxiv, 

 p. 210. 



