Prismatic Structure in Sandstone, 8fc. 263 



and other circumstances, one or other of these substances. 

 That the effects there enumerated take place in innumerable 

 instances is fully demonstrated. That they sometimes do not 

 occur where they might be expected is equally admitted. But 

 it is not necessary to show reasons for these exceptions, as the 

 discussion would be here out of place, and has often been 

 before the public in many different forms. 



It is therefore proved by natural appearances, or in the great 

 experiments of nature, that the action of heat existing in the 

 trap rocks is capable, among other changes, of converting red 

 argillaceous sandstone, and ferrugino-siliceous shales into 

 jasper, and the latter, under other modifications, into ironstone. 

 It was also shewn in the first part of this paper, that the action 

 of artificial heat was capable of producing the prismatic struc- 

 ture in sandstone. 



It remains to produce an instance in nature where the pre- 

 sence of trap and the prismatic form in the sandstone near it 

 concur ; and this example is found in the instance in Rum 

 above mentioned. It will probably be found in many other 

 places, when the facts and reasonings which this paper now 

 promulgates for the first time shall become known to geologists. 

 As the action of heat, therefore, or the presence of trap, and 

 its presumed influence over the strata, have been shewn capable 

 of producing, separately, either the prismatic configuration, or 

 the change to jasper in certain modifications of sandstone, and 

 that instances of each of these have been adduced, it does not 

 seem at all illogical to conclude, that the same cause may pro- 

 duce both these effects united. I must regret that I cannot 

 produce an actual instance of the contact of trap with this 

 union of the jaspideous character and the prismatic structure, 

 but future researches will probably discover them. 



In defect of this, we can only be guided by reasonings from 

 analogy, in attempting to explain the causes of the peculiar 

 nature of the three examples described in the second part of 

 this paper. 



The case of Rum is indeed proved; since, as already 

 observed, the trap is found in contact with the prismatic sand- 

 stone. In Arran, the presence of trap, in various parts of the 

 tract in which the prismatic ironstone occurs, is evident $ and 



