Report of the Meetings Volume 26
This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1857 edition. Excerpt: ...be laid on two supports parallel to and equidistant from the strike edges, it may be found that at one of these edges fracture will be more easy than at the other. Then turn over the slate to see if the facts will be reversed, and the other edge give the easiest fracture. An observation in the aflirmative is in my note book, for 1836, at Llanberis. I shall be glad to know if it has been noticed by others in this or other localities. Hence it appears probable, that besides the principal cleavage, some slates contain a secret lamination, or ' secondary cleavage, ' which occasions a partial fissility; but in general this kind of structure produces no such distinct appearances in the blocks and masses as to be often recognized on a great scale. Some cases in which I had supposed such a structure to be real and important, turned out on further research to be merely examples of symmetrical jointing. Prof. Sedgwick, however, refers me for satisfactory instances to the old black slates of Buttermere, and to the vicinity of Yspytty Evan, in North Wales. One of the cases in which a second set of cleavage planes was supposed to cross the principal cleavage frequently and regularly, is the "pencil bed " of Skiddaw slate dug in Westmoreland, near Shap. Mr. Sharpe has examined this curious rock, and finds in one case (Thornthwaite Gill) the principal cleavage parallel to the original beds and dipping N.W. 60; the secondary cleavage crosses it nearly at right angles and dips S.E. between 20 and 30. In another case (Rosgill Moor) the beds dip N.E. 30; the principal cleavage N. by W. 60; the secondary cleavage S. by E. 15. By natural decomposition, small square prisms are produced, whose sides measure one-quarter to half an inch...
- 189 x 246 x 18mm | 617g
- 13 Sep 2013
- United States
- black & white illustrations