146 Boulders of Trap Rocks, 3,-c, 



composed of ancient lavas. In that part of our island now 

 under review, the surface being unbroken save by the partial 

 sinkings for water and gravel, there does not appear such ex- 

 citement to the study of geology as in the mining and quarry- 

 ing districts, or where the surface is broken by natural causes, 

 displaying the rocks to the discerning spectator in the most 

 inviting manner. Still, to enquiring individuals, the deposits, 

 even in these localities, become deeply interesting when their 

 mineral chracters are investigated and made known. 



Numerous other rocks in the series have contributed 

 towards the great accumulation of detritus which forms our 

 deposits of gravel : but the chief object of the present 

 communication is a confirmation, by a series of experiments, 

 that the boulders of basalt, porphyry, &c. s belong to that class 

 of rocks termed trap, and to which an igneous origin is attri- 

 buted by our most eminent geologists. 



To prove, as far as artificial heat on a small scale can ac- 

 complish it, the igneous nature of the basalts, porphyries, 

 greenstone, and even granite, which I have found in our 

 neighbourhood, I have recently submitted fragments of each 

 of those rocks to the action of heat produced by the oxy- 

 hydrogen blow-pipe on Hemming's principle ; and I have 

 much pleasure in stating that they were all completely fused 

 by this means, without the aid of any additional flux. While 

 under this process, some melted much more readily than 

 others : the basalts, for instance, melted comparatively easily 

 to what the porphyries did : this difference in fusion appears 

 easily to be accounted for, by the basalts containing iron, 

 soda, or potash (perhaps all three), more or less, in larger 

 quantities than is found in the porphyries. 



The compact felspar, which forms the matrix of the porphy- 

 ries, containing potash, may be the means of facilitating its 

 fusion ; but its not containing so much iron as the basalts, 

 may be the reason of its fusing more slowly than the latter. 



All the basalts which I submitted to this test, in this series 

 of experiments, melted with less than half the heat that was 

 employed to fuse the porphyries. 



The following are the particulars observed as the fusion 

 of each specimen mentioned in Vol. VIII. took place. The 

 fragments submitted were about the size of peas, and are now 

 attached to their respective specimens. 



1> Hornblende rock. This is merely basalt accompanied by 

 minerals frequently found in this kind of rock. This readily 

 fused into a black glass, with effervescence. 



2. Hornblende basalt. This fused with rather more diffi- 

 culty, and with effervescence. 



