as exhibited in the Devonian Limestones of Devonshire. 27 



If a thin section of an organic sand be examined with the 

 microscope, it will be seen that the fragments of coral and shell 

 are usually much longer than broad, as if derived from sections 

 of more or less flat portions. If the rock be very thin-bedded, 

 for instance like the Stonesfield slate, the greater part of these 

 lie in the plane of stratification ; but in such thick-bedded rocks 

 as occur in the districts under consideration, when not cleaved, 

 this is not the case, except with very large fragments. The 

 smaller have their longer axes inclined in all positions, so that 

 when a section cut perpendicular to the stratification is examined, 

 there is seen to be no such arrangement as to give rise to any 

 decided line of weakness, along which the rock would split in 

 preference to any other. Let us now inquire what would be the 

 effect on such a structure if the dimensions of the rock were 

 changed by mechanical pressure. 



If a rock has not been compressed, we may express this by 

 saying that the ratio of the altei'ation in any two directions at 

 right angles to each other is as 1 : 1 ; whilst if it had been com- 

 pressed in such a manner that the proportion between lines of 

 equal length before compression was changed so that in the line 

 of pressure the length was one-sixth of that perpendicular to it, 

 we may say that the ratio is as 1 : 6. If, for instance, before 

 compression we had a circle, afterwards it would be an ellipse, 

 whose axes were as 1 : 6. This is a very common amount of 

 change in the line of dip in rocks that have a good cleavage, and 

 so I take it by way of illustration. If, then, before compression 

 a long-shaped fragment was inclined at any given angle to the 

 plane perpendicular to the subsequent pressm-e, it may be seen 

 from general mechanical principles, as well as proved by actual 

 experiment, that the tangent of the angle, after the change of 

 1 : 6, would be one-sixth of the tangent of the original angle. 

 Thus we have — 



Angks of Inclination to the plane pei-pendicular to the pressure. 



