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



the fragments when they were first deposited ; a fact that may be 

 seen in progress in some recent Umestones. In my descriptions 

 I shall exclude those that may be seen by the naked eye to con- 

 tain much of corals or other large organisms, and only take into 

 account the more homogeneous deposit surrounding them, con- 

 stituting a limestone of which the true nature cannot be ascer- 

 tained by a mere inspection of the rock in its natural state. 



In the following analyses I distinguish the different specimens 

 by the names and numbers of my microscopic objects. As an 

 example of an organic sand of Devonian age, I select one from 

 near Torquay. 



Hope's Nose No. 1. (Physical analysis.) 



Detached minute joints of encrinites, not fractui-ed \ .q,q 

 or worn J 



Fragments of other organisms, chiefly corals . 8' 7 



Organic clay, existing as dirt in the cavities of the"! g.-i 

 coral, &c J 



Crystallized calcareous spar, infiltered into the cavi- \ . g.q 

 ties between the organic fragments .... J 



1000 



The fragments are from '01 to "05 of an inch in diameter, 

 but chiefly from '02 to -03. The amount of infiltered calcareous 

 spar agrees very well with what calculation indicates as being 

 very probable for that of the cavities in a deposit of this cha- 

 racter. 



As an example of an organic sandy clay, I give one from near 

 Plymouth. 



Plymouth No. 3. (Physical analysis.) 



Fragments of coral . . . . 12*3 



Portions of encrinites . . 11 '5 



Organic clay 25 '7 



Crystallized calcareous spar. 50*5 



100-0 



The organic fragments are from -01 to -05, but on an average 

 from '02 to '03 of an inch in diameter. The crystallized calca- 

 reous spar is chiefly such as was derived from the crystallization 

 of the organic clay. 



There are many limestones that contain grains of quartz and 

 other minerals. As an illustration of this I give one from near 

 Ilfracombc, which may be called a quartz-sandy organic clay. 



