as exhibited in the Devonian Limestones of Devonshire. 33 



axes at least as 1:4, and are just of sucli a character as 

 would occur from the compression of the jomts to the extent 

 which the alteration of their own form indicates. I could de- 

 scribe similar facts with respect to other organic bodies, but 

 these appear to me so very clear and decisive that I think it 

 unnecessary. In most cleaved limestones this compression of 

 the solid organic fragments is only slight, and merely their posi- 

 tion is altered ; apparently on account of the materials by which 

 they are surrounded having given way more readily than their 

 own tissue ; but yet it may be clearly seen that, other circum- 

 stances being the same, the amount of their compression is in 

 proportion to the perfection of the cleavage. 



Not only are the organic fragments thus compressed^ but also 

 crystals of calcareous spar and dolomite. In uncleaved lime- 

 stones detached perfect rhombic crystals are often seen ; and in 

 some highly cleaved there are such forms as would result from 

 the breaking up and compression of similar crystals. Moreover, 

 in uncleaved limestones the calcareous crystals, filling the cavities 

 in organic bodies or derived from the crystallization of organic 

 clay, have their crystalline cleavage planes almost invariably 

 straight; whilst in cleaved limestones they are often veiy con- 

 siderably bent ; and this is so particularly the case in the very 

 highly cleaved specimen from Kingskerswell, that there is scarcely 

 any calcareous spar that has straight cleavage planes, and much 

 of it is so broken up and bent that it is only by comparing in- 

 termediate examples that the true nature of the structure can be 

 ascertained. This then clearly proves that the compressing force 

 acted so intensely and so gradually as to change the molecular 

 arrangement even of calcareous spar and bend it, in the same 

 manner as we may by the hand easily bend such flexible, 

 unelastic crystals as those of talc or lead. It may perhaps be 

 well to state that I do not think that this indicates that the rock 

 was softened or melted by heat, but that it changed its form 

 like a malleable substance, by the gradual movement of the ulti- 

 mate atoms one over the other. 



This compression and molecular rearraugment, combined with 

 change of position, has been most effective in producing a line of 

 weakness and cleavage in the finely crystaUized organic clay of 

 cleaved limestones. When a thick-bedded, uncleaved, fine- 

 grained organic clay, such as the white lias of Radstock near 

 Bath, is examined in a section cut per- 

 pendicular to the stratification, the struc- 

 ture is like fig. 5, which is magnified 300 

 linear. The stratification is in the line 

 of the length of the figure, and, as will 

 be seen, the longer axes of the crystals are 

 arranged promiscuously, without relation 



Phil, May, S. 4. Vol. 11. No. 69. Jan. 1856. 



