ly having 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 had this day witnessed. A great degree of heat applied 
to granite transforms it into pitchstone, the constituents of which are nearly or 
quite identical—the difference between the two substances being mainly in me¬ 
chanical aggregation. Pitchstone is readily fused into glass, and was formerly 
used in France for that purpose. It had been objected to the igneous theory that, 
if heat were the principal agent of the changes which the surface of the earth has 
undergone, carbonate of lime could not have been found in the trap formations, 
as heat would drive off 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 heat. Instead of being decomposed, it will be changed into crystallised 
carbonite of lime. And, in conclusion, he stated in general 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 has been 
called by one of them “a perfect jewel.” It was here Hutton obtained the first 
crude notions of his igneous theory, now so almost universally received; and here 
he reduced them to form, and gathered arguments to sustain that system. The 
small circuit of the island—its strongly-marked anticlinal line--—its well-defined 
stratification—its embracing nearly all the recognised series of rocks in the regular 
order of their superposition, and the presence of numerous forms of characteristic 
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. 
SeJgewick and Murchison, after a careful examination of the island—having been 
governed by the well-known order of superposition—the mineralogical nature of 
the rocks, and the characteristic 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 refer the lower conglomerates to the old red sandstone , 
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 of concretionary limestone, similar to the cornstone (cornitifer- 
ous limestone, as Professor Eaton would call it) of Herfordshire and South Wales. 
The central group they refer to the carboniferous series, because it contains beds 
similar to the mountain limestone, having the same suite of fossils, and because 
these beds are overlaid by a carboniferous deposite embracing three or four of the 
most characteristic fossils of the true coal measures. The minerals and the greater 
part of the fossils of the upper red limestone would tend to bring them, also, within 
the carboniferous order; and it is well known they elsewhere alternate with the 
coal measures; and, if I mistake not, such is the case with the Cumberland 
(Maryland) coal fields. They admit, however, that many of the fossils belong to 
the magnesian limestones overlying the coal measures of Durham. 
