Page:The Journal of geology (IA journalofgeology11893univers).pdf/267

This page has been proofread, but needs to be validated.
THE BASIC MASSIVE ROCKS, ETC.
693

belonging with the Keweenawan rocks, the Wisconsin mass was nevertheless recognized by Irving as presenting "the appearance of a certain sort of unconformity with the overlying beds. These gabbros, which lie immediately upon the Huronian slates, form a belt which tapers out rapidly at both ends, and seems to lie right in the course of the diabase belts to the east and west, since these belts, both westward toward Lake Numakagon, and eastward toward the Montreal river, lie directly against the older rocks, without any of the coarse gabbro intervening."..."The great extent of coarse gabbro in Minnesota seems to sustain somewhat the same relations to more regularly bedded portions of the series."[1]

The only other descriptions of this great gabbro mass are to be found in the reports of the Minnesota survey. In the report for 1887 Prof. N. H. Winchell[2] details a few of his observations on the "great gabbro flood," and surmises that the "flow" did not escape through a single fissure. The structure of the rock is reported as roughly columnar, with sometimes apparent indications "of the existence of imbricating layers having a gentle dip, as if the fluid rock had swept over the country in successive tides....In texture the gabbro is characteristically coarse. Sometimes some of the constituent minerals are half an inch in diameter. From this they graduate down to an extreme degree of fineness."

From the macroscopic descriptions of other varieties of the rock that follow it is evident that the writer is not dealing exclusively with specimens taken from the great "gabbro flood" at the base of the Keweenawan, for, as the sequel will show, this is composed of a rock which, in its unaltered state, possesses a remarkably uniform texture, and is so well characterized that any departure from it is presumptive evidence that the rock exhibiting the variation belongs not in the "basal flow," but in some one of the numerous smaller beds interstratified with the Animi-

  1. ↑ Copper-Bearing Rocks, p. 155.
  2. ↑ Geol. and Nat. Hist. Survey of Minnesota, 16th Ann. Rept. for 1887.St. Paul, 1888, pp. 360-362.
This article is issued from Wikisource. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.