Page:The Journal of geology (IA journalofgeology21894univers).pdf/124

The succession in the district, as determined in ascending order, is (1) reddish grey gneiss without distinct signs of bedding or stratification, but with a foliated structure; (2) reddish orthoclase-gneiss interstratified with hornblendic, quartzose, and garnetiferous gneiss and beds of quartzite, the whole showing a well stratified arrangement of beds; (3) grayish and rusty gneiss passing gradually upward into the calcareous portion of the system, between the gneiss and the limestone there being interstratifications of the two; and (4) schistose, sericitic, chloritic, and micaceous schists of the Hastings series. This division overlies the crystalline limestone, and is believed to represent the lower member of the Huronian system. This arrangement of the Laurentian accords very closely with that in New Brunswick, as given by Bayley and Mathew. Unconformable upon the Laurentian of Ontario is the Paleozoic.

Adams^[1] describes the anorthosite of Canada, and gives its relations to the surrounding rocks. The great mass of the Archean of Canada is composed of an orthoclase-gneiss, which is in many places laminated, but is in large part little laminated, and probably of eruptive origin. Much of the laminated gneiss is probably sedimentary. In certain regions the laminated gneiss is interlaminated with crystalline limestones, quartzite, amphibolite, etc. This series is a higher part of the Laurentian, and was called by Logan the Grenville series; while the lower gneiss, which does not bear any of this rock, was called the Ottawa gneiss. The limestone, graphite, etc., are evidences of the existence of life during the deposition of the Grenville series, and this was the earliest life of the planet.

All of the minerals of economic importance occur in the Grenville series. The relations of the Grenville series to the Ottawa series have not been certainly determined, but it is probable that the Grenville series lies discordantly upon the old gneiss, the upper series being sediments originally like those that are deposited to-day.

The anorthosite group, or Upper Laurentian of Logan, is an eruptive rock belonging to the gabbros. It is characterized by a predominance of plagioclase, which frequently is the only mineral of the rock. The rock is hard and originally was completely massive. This original structure has been modified so as to take on an extraordinary cataclastic structure, which has also given the rock a schistose character. This is not ordinary dynamic metamorphism, but is caused by a movement of the rock mass while it was deeply buried and near its melting point.

The anorthosite, although so regarded by Logan, is not a distinct sedi-