140 Henry Clifton Sorby on the 



is seen to be perpendicular to that of the pressure. I have observed 

 numerous suchlike cases, and in fact nearly all the greatly contorted 

 coarser-grained beds in North Devonshire present similar appear- 

 ances. They are, in fact, analogous to what would occur if a strip 

 of paper, for instance, was included in a mass of some soft plastic 

 material, which would readily change its dimensions. If the whole 

 was then compressed in the direction of the length of the strip of 

 paper, it would be bent and puckered up into contortions, whilst the 

 plastic material would readily change its dimensions, without such 

 being the case ; and the difference in distance of the ends of the 

 paper, as measured in a direct line, or along it, would indicate 

 the change in dimensions of the plastic material. 



The green spots so often seen in slate, do also most distinctly indi- 

 cate a similar change of dimensions. I am persuaded that they have 

 been concretions of a peculiar kind, formed round bodies lying in the 

 plane of bedding. In rocks without cleavage, such green spots are 

 almost perfect spheres, or are elongated in the plane of bedding. The 

 facts seen in those in slate rocks, prove, I think, most clearly that 

 they were exactly similar before the cleavage was developed. Now, 

 however, they are greatly compressed in a line perpendicular to 

 cleavage, and somewhat elongated in the line of its dip. When 

 they have been originally spherical, their long axis agrees with 

 the dip of cleavage, both in its plane and perpendicular to it ; 

 whereas, if they have been originally more elongated in the 

 line of bedding, their longer axis is inclined in such a manner 

 as would then occur if they had been subsequently elongated 

 in the direction of dip of cleavage ; that is to say, it does not now 

 coincide with it, but deviates towards the plane of bedding. If, 

 however, the stratification is perpendicular or parallel to the cleavage, 

 the longer axis of the spots does agree with the dip ; or, if it cuts 

 the plane of cleavage in the line of true strike, then also, in the plane 

 of cleavage, the longer axis of the spots coincides with the line of dip. 

 On the whole, all the facts agree most perfectly with what would oc- 

 cur if the spots had originally been similar to those in non-cleaved 

 rocks, and the mass of slate had been greatly compressed in a line 

 perpendicular to cleavage, and somewhat elongated in the line of dip. 



Many of the finer-grained slates used for roofing, contain minute 

 rounded grains of mica, seldom so much as iJ tb of an inch in 

 diameter, and usually much less, which are of nearly the same 

 thickness as width, and not merely flakes. When these are cut 

 through in the thin sections used for microscopical examination, they 

 are seen to be composed of many lamina?. When the line of lamina- 

 tion, — that of the crystalline cleavage of the mica, — coincides with 

 the cleavage of the slate, these rounded grains retain their form unal- 

 tered. If the lamination is perpendicular to the cleavage, the rounded 

 form still remains, but the lamina? are generally not straight, being 

 irregularly bent in just such a manner as if they had been coin- 



