460 ALFRED C. LANE 



stratigraphic term. It is a residual term under which all large old 

 (or ill-known) batholiths or plutonic rocks may be grouped, and I 

 presume will prove very convenient in this elastic sense. I shall 

 use it about as Brooks used it. Those who believe in sub- 

 crustal . fusion will, however, use it, applying it to very much the 

 same rocks, but implying more, that they are softened parts of the 

 basement of the Huronian. It is hardly likely that the term will 

 stay in this stage of definition very long. As the original Laurentian 

 area is more carefully studied and its connections with Lake Superior 

 made out, I presume we shall find out just about what proportion 

 of the original Huronian must be taken from the bottom to be equiva- 

 lent to the original Laurentian, and we should then revert to the 

 original division of the Archean into the Huronian above and 

 Laurentian beneath. 



A few words, in conclusion, as to the top of the pre-Cambrian. 

 Here, as usual throughout the geological column, there is difficulty 

 in fixing the exact line of division. That the main part of the Lake 

 Superior sandstone is Cambrian I have no question. It is, in fact, 

 upper Cambrian apparently. A specimen with trilobites from the 

 Menominee Range, Mr. CD. Walcott determined as of the Ptychaspis 

 zone. Wells at Grand Marais and Lake Linden show that at those 

 points it has over a thousand feet of thickness. 



Dr. Hubbard has proved conclusively to me that it overlaps 

 unconformably on the base of the Keweenawan. 1 But does that 

 imply that the Keweenawan belongs to a different system ? Neither 

 Seaman nor I think so. For while there is the unconformity with 

 the base of the Keweenawan, and the Keweenawan is very thick, this 

 thickness of thousands of feet is composed of rocks which accumulate 

 with extreme rapidity. They are almost wholly sandstones, conglom- 

 erates, and traps. Geologically, the Keweenawan may represent 

 no more time than some of our present volcanoes lasting from Ter- 

 tiary times. Moreover, in such a series intra-formational, and even 

 more intra-systemic discordances may be expected. As a matter 

 of fact, we find bowlders and even agates of the Lower Keweenawan 

 traps in the Upper Keweenawan. Yet there is a steady approxima- 

 tion, (i) in dips, (2) in stratigraphic distribution, (3) in disturbances, 



1 Vol. VI, Part II, pp. noff. 



