36 



ly hnving been highly delighted with the excursion. During the passage back, 

 Professor Johnston lectured on the influence of caloric in producing the changes 

 the evidences of which we liad ihis day witnessed. A great degree of heat applied 

 to granite transforms it into pitchstone, the constituents of which are nearly or 

 quite identical — the dilVereiice bttweon the two substances being mainly in me- 

 chanical aggregation." I'itchstone is readily fused into glass, and was formerly 

 used in France for that purpose. It had been objected to the igneous theory that, 

 if beat were the principal agent of the changes which the surface of the earth has 

 undergon*, carbonate of lime could not have been found in the trap formations, 

 as heat would drive oft" the carbonic acid, and decompose it. Yet it is seen to 

 occur in these very formations. [I have found it, myself, among the traps and 

 whynstone dykes at the Giant's Causeway.] But it is well known that, if press- 

 ure be applied, this result will not be produced. As, for instance : Take powder- 

 ed chalk; put it into a gun-barrel ; seal it hermetically, and apply an intense de- 

 gree of h(^»t. Instead of being decomposed, it will be changed into crystallised 

 carbonite of lime. And, in conclusion, he stated in g-eneral terms that in fact 

 there is no geological phenomenon which may not receive a satisfactory explana- 

 tion from the well-known action of heat. 



The isle of Arran has long been a favorite resort of naturalists, and lias been 

 called by one of them "a perfect jewel." It was here Hutton obtained the first 

 crude notions ot his igneous theory, now so almost universally received; and here 

 he reduced them to form, and g.ithered arguments to sustain that system. The 

 small circuit of the island — its strongly-marked anticlinal line — its well-defined 

 stratification — ii.« embracing nearly all the recognised scries of rocks in the regular 

 order of their superposition, and the presence of numerous forms of characieristic 

 fossils, have all combined to invest it with peculiar interest for the student of Na- 

 ture, and more especially for the lover of geological investigations. Messrs. 

 Sedgewick and Mnrchison, after a careful examination of the islami — having been 

 governed by the well-known order of superposition — the mineralogical nature of 

 the rocks, and the charanteristic fossils — arrived at certain conclusions, which they 

 have most ably summed up, but which, in the short limits of a letter, I can only 

 succinctly notice. They reler the lower conglomerates to the old red scnidstonr, 

 because they are subordinate to beds representing the carboniferous order — because 

 they contain beds possessing the characteristics of grauwacke, and because they 

 have inferior beds oV concretionary limestone, similar to the cnrusfonc (cornitifer- 

 ous limestone, as Professor Eaton would call it) of Herfordshire and South Wales. 



The central group they rofer to the carboniferous series, because it contains beds 

 .•limilar to the mountain limestone, having the same suite of fossi|.<i, and because 

 thcbc betls are overlaid by a carboniferoux dcpoi-ili' embracing three or four of the 

 most charactf ristic fossils of the true foal measures. The minerals and the greater 

 part of the tossils of the upper red limc.>;tiine would tend to bring thom, also, within 

 the carbonil"eroui order; and it is well known tiiey eUewhere alternate with the 

 qpal measures; and, if I mistake not, such is the ca.te with the Cumberland 

 (Maryland) coal ticlds. Tlicy admit, however, that tnaiiy of the fossils belong to 

 the magiicoiiaii liMic>luncs overlying the coal ineasurco nf Durham. 



