490 



REPORT — 1856. 



the structure of rocks by the microscope, and especially by the use of thin 

 sections, he has applied this method of research to ascertain the origin of 

 slaty cleavage. In the course of a careful examination of contortions in North 

 Wales and Devonshire, he was convinced that they indicate a very con- 

 siderable amount of lateral pressure, the thickness of the contorted beds being 

 very different in one part to what it is in another (see Diagram 25). In the 

 case referred to, the amount of compression inferred is so great, that points 

 which appear to have been 38 inches apart, are now at the distance of only 

 9 inches. Unyielding parts have been contorted, yielding parts simply pressed 

 together in one direction and extended in another. The green spots so often 

 seen in purple slate, also indicate great change of dimensions in the mass. In 

 rocks without cleavage they appear spherical ; in cleaved slates they are found 

 to be compressed in the perpendicular to cleavage, elongated in the line of its 

 dip ; so that, if originally spherical, they have become ellipsoids of three 

 dimensions, the shortest axis lying across the cleavage, the longest in the 

 line of cleavage dip, while the third axis of intermediate length coincides 

 with the strike of the cleavage. These three axes, in a case not supposed to 

 be extreme, though doubtless above the average, in the slates of Llanberis 

 and Penrhyn, are found as 1 : 3'75 : 6 ; from which it follows that the sphere 

 has been compressed to less than half the original bulk (as 3'75^ to 

 1 X 3-75 X 6-0), or as 100 to 43. 



In a mass so compressed, the relative angular positions Fig. 37. 



of all the particles not exactly perpendicular to the line 

 of pressure or exactly parallel to it would be changed. 

 Supposing the particles, or some of them, to be unsym- 

 metrical (as they mostly are in the brecciated slates, and 

 indeed in most kinds of slate), and that their lengths 

 were equally presented in all directions, or inclined at 

 all angles to the plane perpendicular to the line of pres- 

 sure, — we shall find after compression their inclination 



6' by the formula tan 0'=- 



where c is the ratio of 



the longer to the shorter axis of the ellipse representing 

 the compression, 6 the original angle, and d' the angle 

 to which it has been changed by compression. In the 

 case assumed above c=6, where and 6' appear in the 

 following Table : — 



After compression. 



Originallv. 

 )= 0° 

 10 

 20 

 30 

 40 

 SO 

 60 

 70 

 80 

 90 



0'= 



Or suppose in a small part of the original mass the particles to be so dis- 

 tributed as to occasion ten planes of equal fissility, having the same strike, 

 and surrounding the same axis, and inclined to one another 10°, — this part 

 of the mass, after undergoing compression c (6:1), would still possess ten 

 cleavage planes, but they would be inclined to one another as in Diagram 38, 

 which corresponds to the calculation just given. 



