Prismatic Structure in Sandstone, Sfc. 251 



but that is of such a nature, that the lens, at least, is incapa- 

 ble of determining what it really is. It is necessary yet to 

 observe, respecting the character of this burnt sandstone, that 

 instead of being white, as in its original state, it is of a grey 

 colour: and, on examining it with the lens, it is easy to see 

 that particles of dark grey and transparent quartz are inter- 

 mixed with the colourless and transparent ; so as to produce 

 this effect. The nature of this change in the quartz is not 

 intelligible ; and the specimen presents no other appearances 

 that I could discover, capable of throwing any light on the 

 subject. 



It had formerly been proved, by the occurrences so common 

 in glass-houses and iron-furnaces, among the glasses and slags 

 which have remained long in the fire, that, in a long continued 

 heat, or rather during a process of slow cooling, these sub- 

 stances were capable of crystallizing, so as, on solidifying, to 

 lose their ordinary character of glass, and to assume the crys- 

 talline arrangement found in many kinds of rock. The expe- 

 riments of Mr. Watt and others, on stones fused and treated in 

 the same manner, confirmed their accidental results. But it 

 remained to be proved that heat applied to rocks in a degree 

 inferior to that which is required to effect their fusion, was 

 capable of changing in an analogous manner the internal 

 arrangement of their particles, as happens in the case of some 

 other solid bodies. This was the object of the experiments 

 which, as already mentioned, I undertook without success, but 

 which this accidental experiment has proved to be possible. 



But the change, in this case, is not a change of that which 

 is properly to be considered the crystalline arrangement of 

 particles. There appears to be no such arrangement in sand- 

 stones ; nor are there any of the characters of true crystalliza- 

 tion in the prismatic structure which has followed the action 

 of the heat in this instance. 



What the real nature of this analogy between the crystalline 

 and concretionary structure in rocks may be, it is impossible 

 to conjecture, ignorant as we are of the real nature of both 

 these processes. In crystallization, we can see that the crys- 

 tals, assuming the integral form or molecule, is the most simple, 

 consists of similarly formed parts, divisible, as far as we have 



OCT.— dec 1829. S 



