as exhibited in the Devonian Limestones of Devonshire. 31 



great mass of the beds. They indeedj on a very small scale, 

 pi'esent us with the same phsenomena as are seen in the green 

 spots in many of the Welsh slates, as briefly described by me in 

 the paper already referred to. In Ilfracombe No. 8, whose con- 

 stitution is given above, which is an organic clay, foliated with 

 lenticular portions of dolomite, it is seen that the patches of 

 crystals have been compressed and elongated in a similar 

 manner, and hence they lie, not exactly in the plane either of 

 stratification or cleavage, which are inclined to one another at 

 from 10° to 20°, but in such a direction as is the resultant of 

 their combined influence. In a section in the plane of cleavage, 

 the elongation in the line of dip is most clearly exhibited. 



In all the cases hitherto considered the organic fragments are 

 not very numerous, and are so imbedded in an excess of organic 

 clay, that, when the dimensions of the rock were changed, their 

 position was altered, but they themselves did not suff'er much 

 compression. However, it is very far from being so when there 

 was not much organic clay, or when the compression was very 

 great ; for then they are much altered in form and structure, 

 and sometimes to so great an extent that it was some time before 

 I could ascertain to what kind of organism they belonged. Mr. 

 Sharpe has shown (Quart. Jouru. of Geol. Soc, vol. ii. p. 74) 

 that the larger organic bodies have had their form much altered 

 in cleaved rocks, and I now proceed to prove that the alteration 

 extends in some instances to the ultimate fragments, somewhat 

 in accordance with what he supposed to occur in all slate rocks, 

 as described in his second communication (Quart. Journ. of Geol. 

 Soc, vol. V. p. Ill), no account being taken of change oi position, 

 which, according to my own observations, is a more common 

 cause of the cleavage. For this purpose I select the minute 

 joints of encrinites, whose form and ultimate organic structure 

 is so very determinate and distinct. In an uncleaved limestone, 

 such as Hope's Nose A^o. 1, of which the composition is already 

 given, the joints of encrinites, whose diameter is on an average 

 about 5f'(jth of an inch, have the proportion between their breadth 

 and length as about 3 : 2, though in some cases they are equal, 

 or even as 4:5. Being thus small short cylinders, they give 

 rise to symmetrical sections, some rectangular, and others more 

 or less entire circles or ellipses, whose axes vary on an average 

 from being equal to the proportion of 6 : 7, but when very ellip- 

 tic 2 : 3, or even in rare cases as 3:5. 



Of all the limestones I have examined, one from Kingskers- 

 well near Torquay, a coral-encrinitic sandy clay, possesses the 

 most intensL-ly developed cleavage, so much so that it required 

 unusual precautions in preparing the thin sections ; and it is a 

 most iustructive fact, that, of all I have seen, it also shows the 



