PRE-CAMBRIAN CLASSIFICATION IN ONTARIO 593 



most fully developed, appear to have an approximate aggregate 

 thickness of 29,000 feet.' Elsewhere these sediments are also 

 known to have great thickness. 



The term "Eparchean Interval," concerning which there has 

 been so much discussion in the past, should now be discarded. 

 The so-called Lower and Middle Huronian rocks of the classic area 

 of Lake Huron lie above and not beneath it. There are now known 

 to be two great "intervals" in the pre-Cambrian, that beneath our 

 Animikean and that beneath our Timiskamian, and not merely 

 one as has been assumed. 



Moreover, a dual subdivision of the pre-Cambrian cannot be 

 justified on the basis of differences in metamorphism or other char- 

 acteristics of the rocks. Normally there is a gradual increase in 

 degree of metamorphism from the youngest to the oldest. In the 

 province of Ontario the Animikean rocks are only slightly meta- 

 morphosed or disturbed, while the Timiskamian are disturbed and 

 frequently rendered schistose, and the Loganian are highly meta- 

 morphosed. 



Sediments, Grenville, occur in great volume in the Loganian. 

 Hence the "Archean" cannot be said to be essentially igneous, as 

 assumed at one time. Moreover, ordinary stratigraphic methods, 

 which have been cited as a test as to whether certain groups of 

 rocks should be classed as Archean or otherwise, can be applied 

 in studying the Grenville. This has been shown by the authors' 

 work in southeastern Ontario.^ 



Briefly, it may be said that the discovery of the true relations 

 of the Timiskamian destroys the argument for a dual classifica- 

 tion of the pre-Cambrian. 



COMMENTS ON TABLES I AND II 



The name Keweenawan in the tables is employed in the sense 

 in which it has been used by most writers on the pre-Cambrian 

 of North America. Certain greenstones in the Lake Huron area 

 were classed by Logan as intrusives of Huronian age. They have 

 come to be known as the "Thessalon greenstones," after the name 



' Ont. Bur. Mines, XXIII, 214. 

 = 767:(i., XXII, Part II, pp. 41 f. 



