138 



Henry Clifton Sorby on the 



Professor Phillips and Mr Daniel Sharpe have shewn that the 

 organic remains found in slate rocks indicate a change of their 

 dimensions ; and it was their observations which first led me to 

 test the mechanical theory, as applied to explain the microscopi- 

 cal structure. I am fully prepared to substantiate their observa- 

 tions, and have also ascertained a number of other facts, proving, in 

 an equally conclusive manner, that slate rocks have undergone a 

 great change in their mechanical dimensions, which change is in- 

 variably related to the direction and intensity of the cleavage, and 

 is such that the cleavage lies in the line of greatest elongation, and 

 in a plane perpendicular to that of greatest compression. 



A most careful examination of very numerous contortions of the 

 beds in slate rocks, in North Wales and Devonshire, has led me to 

 conclude that they indicate a very considerable amount of lateral 

 pressure, the thickness of the contorted beds being very different in 

 one part to what it is in another. The accompanying figure will 

 illustrate my meaning, where it will be seen that the thickness of the 

 contorted sandy bed is about four times greater in those parts lying 

 in the mean direction of cleavage than in those perpendicular to it. 



Vertical section seen in the Cliffs near Ilfracombe, North Devon. 

 Scale, 1 inch to 1 foot. 



Fine-grained, dark coloured, shaly 

 slate ; the bedding shewn by bands 

 of coarser grain and lighter colour, 

 which, in the upper part, are not 

 contorted. The cleavage is well de- 

 veloped, and dips about 60° to S. by 



Much contorted bed of coarser- 

 grained light coloured sandy slate, 

 with less perfect cleavage. 



Fine-grained slate, as at the upper 

 part. 



