in the Diluvium of Essex. 149 



which is to be seen in contact with limestone in Glen Tilt, 

 Scotland, fused with effervescence, and swelled up into twice 

 its natural size whilst under its greatest heat. A greater 

 degree of intumescence was observed in this felspar than in 

 all the trials made. It fused into a perfectly white globule, a 

 transparent glassy enamel. 



A small portion of the red granite, also, from Glen Tilt, 

 fused into a white enamel. 



Basalt. — A black basalt from a dyke, which cuts through 

 the porphyry on the north-eastern side of the Calton Hill, 

 about 14 ft. in thickness. 



Another specimen of basalt, from the overlying mass which 

 forms the summit of Salisbury Crags. A finer-grained 

 basalt, from another part of this hill. A specimen from 

 Arthur's Seat. Another basalt, part of the rock on which 

 Edinburgh Castle is elevated. Two specimens of amyg- 

 daloidal trap, from separate and distinct dykes, seen at St. 

 Bernard's and St. George's Wells, cutting through the coal- 

 measures, and crossing the Water of Leith river at Edinburgh. 



Greenstone from the Hill of Kinnoul, near Perth. A speci- 

 men of basalt from a quarry near Perth. Basalt from a dyke 

 which underlies the slate rocks at Dunkeld, and from the rock 

 on which stands Dumbarton Castle ; also several specimens 

 of basalt from the vicinity of this latter town, of various 

 mineral composition, but principally hornblende; with several 

 varieties of basalt from the Island of Mull. 



All the above basalts, with the exception of those two 

 from the dykes crossing the river called the Water of Leith, 

 fused with the greatest ease, with a comparatively low degree of 

 heat, and with a brisk effervescence, into black glass globules. 

 The two specimens which form the exception to this easy 

 melting appear to be of a different composition to the others : 

 they are of a coarse granular structure, blue, and not of a deep 

 black colour, which distinguishes the other basalts from Scot- 

 land. These two basalts fused, with more difficulty, into 

 irregular vitreous masses. The absence of hornblende pro- 

 bably makes the difference. 



From the conclusions drawn from the observations made 

 during the time these operations were proceeding, there is 

 evident a general feature with regard to basalt, those broken 

 from the rocks in situ, and also those from the gravel ; in both 

 instances the fusion was almost uniformly complete : but all 

 the basalts were fused, even the most stubborn of them (so to 

 speak), with much less time and heat than was necessary for 

 the porphyries. The porphyries collected from the gravel, 

 and those which were broken from the same kind of rocks in 



M 3 



