In this paper, which is a revision and extension of one published in the American Geologist of July, 1890, the writer proposes to bring forward some observations on the nature of the contact between the Huronian rocks of Lake Huron, described by Logan and Murray, and the Laurentian gneisses, which it is thought have an important bearing on the question of the origin and relative age of the latter.
The rocks of the Huronian area to the north of Lake Huron are made up of a series of quartzites, graywackes, slate conglomerates, clay slates hydromica, chloritic, and hornblendic schists, greenstones and. some bands of cherty limestones or dolomites. Frequently these rocks show a pyroplastic origin, and tuffs and breccias are very commonly met with. The majority of the clay slates have the appearance of being very little altered, except in contact with the greenstone and other . . .