Henry Clifton Sorby on the Origin of Slaty Cleavage. 137 



cipitated along with it, is very apt to aggregate together, or 

 even fuse during the incineration (if this is not very carefully 

 conducted), and, consequently, is less easily acted upon by the 

 air during incineration ; this must be avoided, and the oxide 

 should also not be allowed to absorb hygrometric moisture 

 before or during weighing. 



It will be found most convenient to add the carbonate of 

 ammonia to the incinerated sulphuret in the same crucible in 

 which it had been ignited, or rather to cover the ignited 

 sulphuret with four or five times its volume of this salt, and 

 then by means of a small agate pestle or glass rod to break 

 up all grains and mix it well together by trituration, which 

 can be easily effected without any loss whatever, as the super- 

 stratum of carbonate of ammonia effectually prevents any 

 particles flying over the side of the crucible. This, with its 

 cover loosely placed upon it, is now very gently heated, until 

 nearly all ammoniacal salts are expelled ; then the heat is in- 

 creased for an instant, and the whole, after cooling over sul- 

 phuric acid, is weighed and estimated as usual. 



On the Origin of Slaty Cleavage. By Henry Clifton 

 Sorby, F.G.S. Communicated by the Author. 



For several years I have devoted myself almost entirely to inves- 

 tigating the physical structure of rocks, both on a large scale, as 

 seen in the field, and by preparing sections of extreme thinness, 

 capable of being examined with the highest powers of the micro- 

 scope. This latter subject has hitherto attracted little or no atten- 

 tion, though the inspection of two or three thin sections will some- 

 times solve most important geological problems. Amongst other 

 branches of the study, I have applied this method of research to 

 ascertain the origin of slaty cleavage, which, being obviously due to 

 some peculiarity of structure, I thought might, in all probability, be 

 solved by that means. The examination of thin sections of slate 

 rocks with high powers, and a comparison with those of similar 

 mineral composition not possessing cleavage, have led me to form a 

 theory to account for their difference of structure, materially different 

 from any yet propounded, and which, in my opinion, not only does 

 so most satisfactorily, but also explains perfectly every fact that I 

 am acquainted with, connected with the subject. To enter fully 

 into the whole would require a long treatise, and I shall therefore, 

 on the present occasion, merely give a short outline of my general 

 conclusions. 



