. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 750 SLATE ROCK OF NORTH DEVON. [Ch. XXXVI. Fig. T60. being as much as a quarter or even half an inch. The hard shells are not affected, but only those which are thin.* Mr. D. Sharpe, follow- ing up the same line of inquiry, came to the conclusion that the present distorted forms of the shells in certain British slate rocks may be accounted for by supposing that the rocks in which they are im- bedded have undergone compression in a direction perpendicular to the plane
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 750 SLATE ROCK OF NORTH DEVON. [Ch. XXXVI. Fig. T60. being as much as a quarter or even half an inch. The hard shells are not affected, but only those which are thin.* Mr. D. Sharpe, follow- ing up the same line of inquiry, came to the conclusion that the present distorted forms of the shells in certain British slate rocks may be accounted for by supposing that the rocks in which they are im- bedded have undergone compression in a direction perpendicular to the planes of cleavage, and a corresponding expansion in the direction of the dip of the Subsequently (1853) Mr. Sorby demonstrated the great extent to which this mechanical theory is applicable to the slate rocks of North Wales and Devonshire,! dis- tricts where the amount of change in dimensions can be tested and measured by comparing the differ- ent effects exerted by lateral pres- sure on alternating beds of finer and coarser materials. Thus, for exam- ple, in the accompanying figure (fig. 760) it will be seen that tne sandy bed dfy which has offered greater resistance, has been sharply con- torted, while the fine-grained strata, a, b, c, have remained comparatively unbent. The points d and / in the stratum df must have been origi- nally four times as far apart as they are now. They have been forced so much nearer to each other, partly by bending and partly by becoming elongated in the direction of what may be called the longer axes of their contortions, and lastly, to a certain small amount, by condensa- tion. The chief result has obviously been due to the bending; but, in proof of elongation, it will be ob- served that the thickness of the bed df is now about four times greater in those parts lying in the main direction of the flexures than in a plane perpendicular to them ; and the same bed exhibits cleavage-planes in the direction of the greatest. (Drawn by II. 0. Sor
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectgeology, bookyear1868
