Principles of stratigraphy,

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. 1913 Excerpt: ...flat river pebbles suffer chiefly a horizontal rotation, so that the top and bottom of the pebbles are rubbed against the overlying and underlying ones, while at the same time the edges are worn off, the result being a mass of flat, worn cakes with smooth, rounded edges. In contradistinction to these observations and deductions, Penck emphatically insists upon the rolling of pebbles on the stream-bed as the chief method of river transport. Such rolling may affect scattered pebbles or the entire mass, in which case the entire sediment on the river bottom is in motion, the individual pebbles rolling and striking against one another, with the result that rounded pebbles are produced. In portions of the Rhine bed, such a mass three meters in depth is thus moved along. (Penck-41-.284.) It is extremely doubtful if the distinctions made by Suess and Hoernes can be considered of more than local applicability. The character of the rock which has furnished the material is probably of much greater significance, as pointed out by Walther. Thus, on the shores of Lake Michigan, where the bed rock is a uniform grained limestone, the pebbles are chiefly of a rounded or rollershaped character, while on Lake Erie, where the cliffs are of shale, flat gravel predominates, except where glacial deposits have formed a local source of supply. Again, on a relatively steep shore, where wave-work is pronounced, as on the northern Massachusetts coast, the pebbles are well rounded through rolling, while on a shallow coast, where the wash of the waves rushes up and down the beach as a sheet flood, the pebbles are more often merely moved backward and forward without much overturning, or, again, the pebbles are scarcely moved, but polished and worn by the sand carried back and forth acros...