Page:A Treatise on Geology, volume 2.djvu/132

 118 of more and less argillaceous rock manifest cleavage, but not in parallel planes, and when this happens, the angular difference of the planes is such that those in the finer grained or more argillaceous bed meet the planes of stratification at more acute angles than those do which traverse the coarser and more sandy or more indurated bed. An example of this in secondary cleavage ("bate") is given in the author's paper on Craven rocks (Geol. Trans. 1828). He has since collected examples more obviously dependent on the difference of the mineral quality of the adjacent beds. Mr. Sharpe has admitted this peculiarity (Geol. Proc. 1848).

The above diagram shows a remarkable case observed in old red sandstone near Cork, (1843); s being soft red marly beds, h harder beds, l a laminated sandstone without cleavage, but jointed.

Another of the characteristic phenomena of cleavage was frequently presented to us, while surveying (in 1839) the Palæozoic strata of North Devon. Surfaces of stratification are usually found to be ridged and furrowed by the edges of cleavage, in such a way that the