Page:The Journal of geology (IA journalofgeology21894univers).pdf/125

mentary geological formation, but it is discordantly upon the gneiss of the Laurentian by intrusion. Its intrusive character is shown by the following facts: it is a plagioclase gabbro; it cuts across the Laurentian schists; it holds as inclusions blocks of gneiss; about its masses forming girdles are many characteristic contact belts. The areas of anorthosite are isolated, and lie along the border of the Archean continent of that time exactly as the volcanoes of to-day are along the continental borders. In the great interior area of Laurentian no anorthosite has been found. The formation is all pre-Cambrian, as shown by the fact that it lies unconformably below the Cambrian. Also before the Cambrian was deposited it received its metamorphism, and was deeply eroded. Its relations to the Huronian have not been determined, but it probably does not belong to the Huronian period, but rather to the closing part of the Laurentian.

The several regions of anorthosite are described separately, that of Morin and Saguenay being most fully considered. The Morin area is surrounded by the Grenville series. In the Grenville series are interlaminated limestones, bands of which can be traced many miles. Within the limestone are frequently thin layers of the gneiss. The limestone is less resistant and more plastic than the gneiss. As a result of folding, the bands of gneiss have been broken up, producing irregular banded blocks, which are isolated in the limestone in such a manner as to give rise to extraordinary pseudo-conglomerates.

The Saguenay region is of great size, 5,800 square miles. It is surrounded on all sides by the orthoclase-gneiss, or Ottawa gneiss. The anorthosite of this district is more basic than that of the Morin district, the plagioclase frequently being labradorite or bytownite. That it is an intrusive is shown by the same facts as in the Morin area.

Comments.—The time of pre-Cambrian life must have been so vast that it is not safe to assume that the rocks of the original Laurentian bear the remains of the first life of the planet. Indeed, it seems probable that the earliest life left in the rocks no permanent evidence of existence. Further, before it can be assumed that the Ottawa Laurentian bears the oldest remains of life, it must be shown that these rocks are older than any other series bearing life remains.

Adams describes the typical Laurentian areas of Canada.^[1] The basement rock here found is the Fundamental Gneiss. It is uniformly reddish or grayish orthoclase-bearing gneissoid granite, poor in mica, and bisilicates. The foliation is often due to movement in a plastic condition. Dark bands of amphibolite