272 PITCHSTONE. 



wise be suspected. Thus it desquamates in solid 

 crusts ; the only change which these have undergone, 

 being the, loss of their colour, causing them to resemble 

 white enamel. This may possibly arise, as in granite, 

 merely from the effects of the atmosphere ; but the 

 same exposure sometimes detects a structure which is 

 unquestionably internal, consisting in waving and in- 

 tricate lines, alternately dark and light ; proving that 

 the rock consists of an intermixture of two distinct 

 compounds, and further, that it has been contorted 

 while in a semi-fluid or flexible state. This fact may 

 be added to its other analogies with the trap rocks, 

 in confirmation of the opinion that, like those, it owes 

 its origin to igneous fusion. 



The ordinary transitions of pitchstorie, besides those 

 into basalt, are into chalcedony, chert, and semiopal. 

 Jt appears also to pass into a substance not easily dis- 

 tinguishable from the conchoidal shining jasper found 

 among the clay strata that are entangled in trap or in 

 volcanic rocks ; with which it appears to have been 

 habitually confounded. It is scarcely necessary to 

 define it further than to say that it is distinguished 

 from all other rocks by its vitreous or resinous aspect; 

 and that it differs from obsidian, with which it pos- 

 sesses in fact no affinity, by its inferior hardness. 



At this stage of such a work, it would be almost 

 an insult to a reader to detail the variations of igneous 

 action whence, with a common origin, pitchstorie dif- 

 fers from the proper trap rocks. A different original 

 material, a quicker cooling, preventing crystal- 

 lization, fusion in situ, and re-heating, will explain 

 every peculiarity, from the independence of the veins 

 to the formation of pearlstone ; and the student will 

 profit more by applying his own reasonings on the 

 chemistry of rocks to these appearances, than if I had 

 occupied some pages in explaining them. 



