30 Mr. H. C. Sorby on Slaty Cleavage, 



change of structure to any crystalline action. The particles are 

 not crystals, but are clearly proved, by their structure and form, 

 to be fragments of organic bodies. If the cleavage was due to 

 the mechanical cause just described, their position is most easily 

 accounted for; but I leave the explanation of the fact, on the 

 supposition of the instrumentality of crystalline forces, to those 

 who advocate that view of the subject. The same is the case 

 with the grains of quartz sand, in such limestones as contain 

 them. For instance, in Ilfracombe No. 5, of which the physical 

 analysis is given above, and in which the cleavage is perpen- 

 dicular to the stratification, as seen in a section cut perpendicular 

 to the cleavage and stratification, the grains of sand are often 

 two or three times as long as broad, and even in some cases five 

 times; but in place of the longer axes lying in the plane of stra- 

 tification, they are chiefly inclined at low angles to the cleavage. 

 I may here remark that there is no difficulty in distinguishing 

 grains of sand from small crystals of quartz, such as are met with 

 in some cherty limestones, and that there is no doubt of them 

 being sand in this, and that their arrangement is due to the 

 cause I have just described. 



When organic clays, with or without larger fragments, have 

 undergone consolidation, it very frequently happens that the 

 crystallization of the granules began in various parts, and forced 

 away the bituminous and other impurities into detached, more 

 or less spherical spaces, which afterwards crystallized in much 

 finer grains. On this account they now appear in sections as 

 darker patches, giving the rock a kind of oolitic structure, though 

 quite distinct from the more genuine oolitic grains, visible to the 

 naked eye. These are in many cases due to crystallization taking- 

 place from certain centres, and this being therefore the reverse 

 kind of process, I call them positive segregatiunal oolites, whilst 

 the others may be called negative. When the limestone has no 

 cleavage, these negative, segregational, oolitic grains, though 

 often of irregular form, are usually more or less equiaxed, and 

 not very much longer in one direction than in another, and the 

 longer axes have no particular arrangement. This is, however, 

 very far from being the case in such as have cleavage. As ex- 

 amples of this I may mention Plymouth No. 3, Ilfracombe No. 5, 

 and Paignton No. 1, of which the composition has been given, 

 and in all of which the cleavage is inclined to the stratification 

 at a high angle. In them these granules, in place of being nearly 

 equiaxed, are so compressed and elongated that they are several 

 times longer in the line of cleavage than in the direction perpen- 

 dicular to it ; and as clearly show that thei-e has been a change in 

 the dimensions of the rock, aff"ecting its ultimate constitution, aa 

 do other facts, seen on a large scale, prove it with respect td the 



