in the Diluvium of Essex. \ 4.7 



3. Compact felspar, with crystals of common felspar (por- 

 phyries). This fused with difficulty, and with effervescence. 

 The colour of the stone is blue; but it fused into a light- 

 coloured mass of irregular forms. This porphyry is com- 

 mon in our gravel pits. 



4 — 6. These three specimens are porphyries; compact 

 felspar with crystals of the same mineral : they are distinct 

 in structure, and their composition is various; but the fusion 

 was nearly alike in all three. They fused with difficulty, and 

 with effervescence. 



7 — 14. These eight specimens are basalts, being composed 

 mainly of hornblende, or augite, in crystals of various forms 

 and sizes: they are all of a black colour. They all fused into 

 perfectly vitreous globules, with a comparatively small degree of 

 heat. It is worthy of remark, that No. 8. was perfectly melted 

 before the flame of the blow-pipe had attained its full size and 

 degree of heat. It has been before observed of this specimen, 

 that, in external form and appearance, this is like the Rowley 

 rag, a stone which the town of Birmingham is paved with; and, 

 if fusion is a good test, they are alike in composition; for a 

 fragment of the Rowley basalt, from one of my cabinet speci- 

 mens, fused in the same time, and with the same low degree 

 of heat, as No. 8. from the gravel. This result is in ac- 

 cordance with the experiment made some years ago, by Mr. 

 Gregory Watt of Birmingham, upon this very stone. He fused 

 seven hundred weightof the Rowley basalt at one time ; and it is 

 stated that it melted with less heat than was necessary to melt the 

 same quantity of pig-iron. Basalt is not scarce in our gravel. 



More than twenty other specimens found in the gravel, 

 consisting of basalt and porphyries, and distinct from those 

 above-noticed, with several kinds of granite, sienite, and 

 greenstone, were in the series of experiments submitted to the 

 action of heat by this blow-pipe. The whole of them were 

 fused, at least fragments of them of about the size of peas; 

 some with much less trouble than others : but fusion was at- 

 tained in all the instances. The basalts fused the most readily 

 in these instances, as well as in the former. 



I then endeavoured to ascertain, by the same test, what 

 analogy existed between the porphyries and basalts found in 

 boulders in our gravel, and the same kind of rocks in situ, 

 in different localities in England and Scotland ; specimens 

 of which I had myself collected, at various times, from the 

 rocks themselves. 



The porphyries selected for these experiments were those 

 from the Calton Hill, Edinburgh, and two varieties of red por- 

 phyry from distinct dykes which cross the river Garry; and are 



m 2 



