as exhibited in the Devonian Limestones of Devonshire. 29 



in my paper in the Edinb. Pliil. Journ. cited above *, I mixed 

 scales of oxide of iron with pipe-clay, so as to have as uniform a 

 structure as could be produced, and then compressed it, when I 

 found that the arrangement was exactly such as that indicated 

 by calculation. This will be better understood from an inspec- 

 tion of the accompanying figures. 



Fig. 1 is a representa- Pi„ o. 



tion of a portion mixed 

 equally, then baked and 

 rubbed to a smooth sur- 

 face; and, like a thick- 

 bedded, uncleaved rock, 

 it has no decided line of 

 weakness, due to the ar- 

 rangement of the parti- 

 cles; whilst fig. 2 is a 

 drawing of a portion ori- 

 ginally of similar struc- 

 tm-e, which, having been 

 compressed, clearly shows 

 that it has been changed, 

 precisely in the manner 

 I have shown by calcula- 

 tion to be a necessary re- 

 sult. The dots indicate 

 lines along which it could easily be split, without there being any 

 fracture of the flakes ; and I think no one could fail to perceive 

 that it would be much easier to cleave it along that direction 

 than in any other, and that a very decided line of weakness has 

 been produced. If then an organic clay, containing fragments 

 of coral or shell, had its dimensions changed in a similar manner, 

 such should be their structural arrangement, and such it really is. 



In studying the cleavage of rocks, it is best to make sec- 

 tions of such as have the cleavage inclined at a high angle to 

 the stratification ; for then there is no fear of confounding 

 them together. I shall first describe the structure in the case 

 of oi'ganic sandy clays. A very good example of this is Plymouth 

 No. 3, of which the physical analysis is given above. In it the 

 cleavage cuts the stratification at about 70°, and the fragments 

 of coral, in place of lying in the plane of stratification, are chiefly 

 inclined at low angles to the direction of the cleavage ; in pre- 

 cisely the same manner as that shown to be the necessary result, 

 if an uncleaved, thick-bedded rock of similar physical constitu- 

 tion had its dimensions altered by mechanical pressure. Now, in 

 such a case as this, it is quite out of the question to refer the 

 * 1853, vol, Iv. p. 137. 



