ON CLEAVAGE AND FOLIATION IN ROCKS. 



385 



Fig. 25. 



perpendicular to the planes of the stratification than the cleavage planes are. 

 Carbonate of lime, or quartz, may often be found filling these cracks ; sul- 

 phuret of iron also occurs in them. The 

 slaty laminae are somewhat twisted about 

 the nodules. 



Mr. Sorby has given us an example 

 (Diagram 25) of the deviation of cleavage 

 planes in passing through a thin bed of 

 indurated gritstone, lying in fine-grained 

 slate near Ilfracombe. The strata being 

 subject to much pressure, the thin grit- 

 stone layer is bent in parallel folds, and is 

 of greatest thickness in the vertices of the 

 folds. In this remarkable case, which is 

 on a small scale, the cleavage laminas in 

 the slate are more or less parallel to the 

 axial planes of the folds ; but in the grit- 

 stone, they deviate into fan-shaped arrange- 

 ments, which on a small scale resemble the 

 laminar structure of Mont Blanc. Here 

 also, as in Diagram 21, the cleavage fissures, 

 on passing through the harder substance, 

 deviate toward a direction perpendicular 

 to its surface. When the axes of the con- 

 tortions of such a bed as g (the hard grit- 

 stone) pass in different directions, the clea- 

 vage invariably passes through the centre 

 of them in planes coincident with the axes. 



This is on a small scale the same law as 

 that already quoted from Professor Rogers, 

 the cleavage plane in each case bisecting 

 the flexures. The author just named pre- 



Fig. 26. 



sents us with a drawing (Diagram 26) very well suited to explain his idea of 

 tan-like cleavage planes, in materials of unlike nature, and bent anticlinally*. 

 * Trans, of Roy. Soc. of Edinburgh, 1856. 

 1856. 2 c 



