The Collected Works of Sir Humphry Davy Volume 6 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1904 Excerpt: ...or depression in the former. That these silicious rocks keep so close company with the older acid rocks is significant as to their genetic relation to the latter. In the more elevated portions of the granite, which would be along the present north shore of Lake Superior, and the region north of the Duluth extension of the Canadian Northern Railway, the deposited chert would be thin, and thinned out altogether where the elevation was more than 1,400 to 1,500 feet above sea-level. That these figures are correct, can be proved by field study in many places where the cherts have been protected by trap overflows; in a few other places, where the latter rock is eroded, black slates still cover them, but in general, those black slates are an insecure basis of proof, occurring, as they do, only locally and not over the whole chert area. Of these thin chert layers a large portion is eroded and carried into the depression southwards. The thickest portion of the chert is south of the above mentioned region, towards the United States boundary, where it rests in a basin to a th1ckness of several thousand feet. This basin rises rather abruptly near the other side of the boundary line, but through its whole extent the ironbearing rocks are absolutely intact, and only a very few feet have been carried away in glacial times. It is easy to recognize which is the uppermost part of these rocks, because below the trap the chert has been melted, usually into a red jasper, which shows a "geflossene" structure, sometimes with the most beautiful arabesques or other fantastic designs. The late Dr. Selwyn, formerly director of the Canadian Geological Survey Department, estimated that in some localities the thickness of the Animikie rocks reach even as much as 12,000 feet. A...