of the Rocks of the Coal Formation. 149 



In these two substances, in addition to silicic acid and alumina, 

 we have a large amount of bases with one equivalent of oxygen, 

 among which protoxide of iron and potash predominate. The rela- 

 tion of silica to the sesquiatomic bases is the same in both, alumina in 

 the blue shale, being in part replaced by peroxide of iron, and though 

 the quantity of monatomic bases in the bituminous shale considerably 

 exceeds that in the blue shale, still the similarity of the two is suffi- 

 ciently striking. It will be observed that, in the above calculation, 

 both substances are represented as anhydrous, while the blue shale 

 actually contains 11 per cent, of water. I infer however from the 

 analogy of the two substances, that the inorganic matter of the bitu- 

 minous shale must also be a hydrated compound, containing the same 

 proportion of water which I have accordingly supposed it to be in the 

 calculation of the constitution of its organic part, given in page 147. 



The inorganic matter of the other rocks is too various to admit 

 of any conclusions being drawn regarding their constitution, although 

 they present some points of interest, which, however, I shall not at 

 present attempt to discuss. 



In conclusion, I may be permitted to hope, that the results of these 

 analyses are sufficiently interesting to lead others into the same field 

 of inquiry, and that from the accumulation of facts, we may eventually 

 be led to some general laws regulating the deposition of the stratified 

 rocks. 



On the Tetramorphism of Carbon. By Henry Clifton 

 SORBY, Esq., F.G.S. Communicated by the Author.* 



The four species into which I find it necessary to divide carbon, 

 may be characterised as diamond, graphite, hard coke, and anthra- 

 cite. They have long been distinguished practically ; the object of 

 the present paper is to explain their various crystalline forms and 

 relationships. 



To commence with the most dense, we have first of all diamond, 

 which has long been known to be crystallized in the regular system, 

 cleavable parallel to the planes of the octahedron, and to have a 

 specific gravity of about 3*52. 



Secondly, we have graphite, plumbago or black lead, which, 

 though usually amorphous, is sometimes found crystallized in regular 

 six-sided prisms, cleavable parallel to the terminal planes. This 

 form of carbon often contains much impurity, and its specific gravity 

 is thereby rendered variable, but when due allowance is made for 

 the ashes, as I shall subsequently describe, it is about 2* 18. 



But, besides these, I find there are two other species, viz. : — hard 

 coke and anthracite. Hard coke, like diamond, belongs to the re- 



* Read before the British Association at Edinburgh, August 1850. 



