Page:The Journal of geology (IA journalofgeology11893univ).pdf/327

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 Lawson, that the anorthosite 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.^[1] 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 Professor Winchell,^[2] although he regards the whole gabbro mass as pre-Keweenawan. 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 gabbro 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 Keweenawan and the upper, more thinly-bedded members of the series was noted by Irving^[3] both for the Bad River area of Wisconsin and for Minnesota, and is