54 



University of California. 



[Vol. 3. 



if there be one or more series of clastic rocks which are of pre- 

 Cambrian age, and which are at the same time post- Archaean, 

 there can be no objection to the application of the term to such 

 rocks. In order to determine this point it will be necessary to 

 define the proximate limits of the Archaean as commonly under- 

 stood. Fortunately this may be done for a large portion of the 

 continent without danger of confusion. About the southern 

 borders of the Archaean terranes of Canada, and about the 

 inliers of the same terranes in the Paleozoic province of the 

 northern United States, the most notable feature in the geology, 

 from a historical point of view, is a profound and persistent 

 unconformity which has been recognized for the last half century 

 as marking off the rocks which Dana named Archaean from the 

 rocks of later age which repose upon them discordantly. This 

 unconformity represents without question an important interval 

 of geological time for which we have as yet discovered no equiva- 

 lent sediments, and for which we have, therefore, no name on our 

 time scale, so wedded are we, even yet, to marking off geological 

 time in terms of sedimentation merely. 



Being greatly impressed with the duration of this period of 

 geological time, although its record is only one of erosion, and 

 being under necessity of referring to it frequently in the sequel, 

 I propose to name it the Eparchcean Interval or Period. 



This Eparchaean Interval intervenes between the Archaean of 

 Lake Superior and the Animikie series, the oldest post- Archaean 

 sedimentary series of which we have knowledge in that region. 

 The Animikie series is followed unconformably by the Keweena- 

 wan series, and this in turn, also unconformably, by rocks of 

 Cambrian age. Here, then, we have two distinct series of rocks 

 which are not only post- Archaean, but post-Eparchaean, which, 

 if pre-Cambrian, might be designated Algonkian; and it was for 

 these very series of rocks, and no other, that the new name was 

 proposed by Irving, which finally took the form of Algonkian, 

 to signalize their supposed pre-Cambrian and certainly post- 

 Archaean age. But even as to their pre-Cambrian age there is 

 still an element of doubt. It is admitted that the Keweenawan 

 is pre-Potsdam, but that is a long way from establishing its pre- 

 Cambrian age. The Potsdam is upper Cambrian, and in other 



