Origin of Slaty Cleavage. 141 



pressed in the direction perpendicular to the cleavage of the slate. 

 Those, however, which lie with their lamination at intermediate 

 angles, as for instance at 30° or 40° to the cleavage of the slate, do 

 not retain their original form, but are broken up and extended out 

 in the plane of their lamination, in just such a manner as would 

 occur if the dimensions of the slate had been changed, as previously 

 mentioned. If carefully drawn with a camera lucida, these broken- 

 up grains can be, as it were, restored to their original form, and the 

 amount of change of dimensions calculated with great accuracy. 



Hence, therefore, in cleaved rocks, whether we examine the 

 diminution in the distance between any two points lying in the line 

 of pressure in contorted beds, the dimensions of the beds in differ- 

 ent parts of contortions, the organic remains, the green spots, or the 

 very minute rounded grains of mica, we find most conclusive evidence 

 of an elongation in the line of dip of cleavage, and of a great com- 

 pression, invariably in a line perpendicular to the cleavage. 



The relation between the compression and elongation varies in 

 different rocks, as would necessarily follow from their different com- 

 position. The examination of the spots on fine-grained, good 

 roofing slate furnishes the best evidence of the absolute condensation 

 in a direction perpendicular to cleavage. If they had originally 

 been spheres, and if there had been no condensation of the rock, 

 but only a change in its dimensions, so that, though its thickness 

 was reduced, it was elongated to a corresponding extent in the line 

 of dip of cleavage, it would necessarily follow that their area would 

 not be changed in the plane perpendicular to cleavage in the line of 

 its dip. Therefore, if, in the plane of cleavage, the length of the spot 

 in the line of dip bore a certain proportion to that in the line of 

 strike, in the plane just mentioned, the ratio of the length of the 

 spot, in the direction of cleavage, to that perpendicular to it, would 

 be as the square of that in the former case. Very numerous and 

 accurate measurements, in the very perfectly cleaved slate near 

 Penrhyn and Llanberis, shew that, in the plane of cleavage, the 

 length of the spots in the line of dip exceeds that in the line of 

 strike in the proportion of 1*6 : 1 ; whilst in the plane perpen- 

 dicular to cleavage, in the line of dip, their length in the line of 

 cleavage is six times greater than perpendicular to it. In the plane 

 perpendicular to cleavage, in the line of strike, the ratio between 

 the length of the spots in the line of cleavage to that perpendicular 

 to it, would be 6 : 1-6 = 3-75 : 1. These results are obtained from 

 so many and various cases, that the effects of bedding, in such regular 

 spots as I chose, would be so slight as not to be of any material 

 consequence. If no condensation had occurred, the ratio of the axes 

 in the plane perpendicular to cleavage would have been as 2*56 : 1, 

 instead of 6 : 1 ; and hence there must have been an absolute com- 

 pression from 100 to about 43. From the nature of the facts, the 

 chances are that it is, if anything, rather too great ; and hence, 



