ANALYTICAL ABSTRA CTS. 3 1 1 



Keweenian is not more than one -tenth of this thickness. Irving's subdivision 

 of the Keweenian into groups, and his estimate of the thickness of various 

 portions of the series are of little value ; a statement which it is as painful to 

 make as it is necessary in the interests of sound geology. The anorthosite is 

 provisionally correlated with the Norian of the Province of Quebec, but as 

 this correlation is merely a hypothesis, the name Carltonian is suggested for 

 this formation. 



Comments : — The main structural conclusion of Letwson, that the anortho- 

 site of the Northwest shore of Lake Superior is older than and was deeply 

 eroded before the deposition of the upper Keweenawan lava flows, seems 

 clearly established, and this is a conclusion of great importance. However 

 the general inferences which are drawn from this relation call for more 

 evidence. 



At the outset it is to be noted that the term Paleozoic is extended to 

 include the Keweenian and Animikie series, a usage not followed by many 

 and involving a great proposition which demands evidence. The question is, 

 however, too large to discuss here. 



The Keweenaw series of Northeastern Minnesota is of great extent and 

 thickness. Irving, in his latest paper on the pre -Cambrian divided the 

 Keweenawan into two divisions, a lower basal gabbro, and an upper series, 

 consisting of thinly -bedded basic and acid rocks.' The anorthosite is but a 

 facies of gabbro, in which the pyroxenic constituent is reduced to a minimum. 

 The most probable explanation of the relations made out by Lawson, as it 

 appears to me, is that the anorthosite exposed on the coast belongs with this 

 great basal gabbro, and this is the position which is apparently favored by Pro- 

 fessor Winchell,^ although he regards the whole gabbro mass as pre -Kewee- 

 nawan. This latter is a matter of definition, and is contrary to the general 

 usage of the term in the past, both divisions having been generally regarded 

 as making up the Keweenawan. The length of the period represented by the 

 Keweenawan was so great, that after the outflow or intrusion of the basal gab- 

 bro there may have been along the Minnesota coast a period of erosion, thus 

 cutting deep into the gabbro, anorthosite and associated rocks. Later in 

 Keweenawan time this eroded surface was covered by the flows of the upper 

 division. Indeed this unconformity between the basal gabbro of the Keweena- 

 wan and the upper, more thinly -bedded members of the series was noted by 

 Irving3 both for the Bad River area of Wisconsin and for Minnesota, and is 



' T/ie Classification of Early Canibrian and pre-Cambi'ian formations, by R. D. 

 Irving. In loth Annual Rep. U. S. G. S., pp. 418-420. 



^ The Norian of the Northwest, by N. H. Winchell. In Bull. Nat. Hist, and Geol. 

 Sur., Minn., pp. 28, 19. 



3 Copper Bearing Rocks of Lake Superior, by R. D. Irving. In Third Annual 

 Rep. of Director U. S. G. S., pp. 134, 136, 137. Also Mon. 5, U. S. G. S., pp. 155, 156, 

 158, 159- 



