Dr. Mac Culloch on Jasper. 67 



or else argillaceous sandstones of different colours ; and it is in- 

 teresting to remark that, both the colours and the texture of 

 the resulting jaspers, vary precisely as might be expected from 

 the nature of these various substances. 



The varieties, accordingly, which originate in fine clay, are 

 generally characterized by a high degree of resinous lustre, and 

 a conchoidal fracture, with a smooth surface. It is particularly 

 important to be able to recognise and distinguish these rocks, 

 as they have often been mistaken for pitchstone, and have given 

 rise to the belief that this substance is occasionally stratified; that 

 it is, in fact, a regularly stratified rock. But the two are essen- 

 tially different, both in their mineralogical characters and their 

 geological relations ; nor, as I shall shew in a succeeding paper, 

 is pitchstone a stratified substance. The prevalence of this im- 

 portant error, and the improper conclusions regarding pitchstone 

 that have been drawn from it, will be very apparent in examin- 

 ing the usual collections of Hungarian or other foreign pitch- 

 stones, in which it will be seen that the greater number of speci- 

 mens consist of this particular variety of jasper. 



In the rocks of the overlying character which appertain to the 

 division of claystone, the progress of induration generally causes 

 them to pass into compact feldspar, as it is (perhaps with no great 

 propriety) called. But, in certain situations, the same claystones 

 are found passing into jasper ; being highly indurated, without 

 acquiring that peculiar character by which in their ordinary 

 states of change, they are characterized. This change seems to 

 occur chiefly among the more ancient rocks of this character, 

 namely, the claystones and porphyries that accompany granite 

 and the older rocks ; and there is no difficulty in tracing its 

 progress, even into the porphyries that so often predominate 

 among this division of the overlying family. Thus there are 

 found porphyries with abase of jasper. It must, however, be 

 apparent, that whenever such transitions exist in the case of any 

 rock, exact distinctions are unattainable ; and many specimens 

 of doubtful character are, therefore, the inevitable consequence. 

 Such jasper is not, however, limited to the overlying rocks 

 when in these situations only ; as it also occurs, in certain cases, 

 F 2 



