Of WHINSTONE and LAVA. 91 



ANALYSIS III. 

 Wbin of the Calton Hilly near Edinburgh. 



The rock of this hill varies much in different parts ; but its 

 general character is that of porphyry. The piece I chofe for 

 analyfis was taken from the fouth fide, about ten or twelve feet 

 below its highefl point ; and was free from calcareous fpar. 



The external characters of this particular piece are as fol- 

 low : It confifts of a greyifli bafis, containing rhomboidal cry- 

 Hals of felfpar of a light reddifh-brown colour ; and fmall fphe- 

 rical mafTes of green earth. The bafis, in its fracture, is uneven, 

 and earthy, and has no luflre. It can be fcratched eafily with 

 a knife, and gives an earthy finell when breathed on. The 

 green earth is foft, and is affected in. fome degree by water ; 

 and being decompofed by the weather, as well as the veins and 

 nodules of calcareous fpar, which are very common in this hill, 

 the rock in many places is extremely porous. 



The fpecimen I have defcribed may be called argillaceous 

 porphyry. It effervefces flightly with acids ; fo that the lime 

 which it contains muft be united to carbonic acid. Its powder 

 is light grey, with a certain fhade of purple. When heated to 

 rednefs, it becomes of a brown colour. It is not attracted by 

 the magnet, either in its natural ftate, or after ignition. By be- 

 ing expofed to a low red heat for half an hour, it lofes 5 per 

 cent, of its weight. It foftens at 44 of Wedgwood. Its fpeci- 

 fic gravity is 1.663, as nearly as I could afcertain, from the ef- 

 fect: which the water had in making it crumble down. 



M 2 One 



