10 Proceedings of the Royal Physical Society. 



perhaps, that of an ordinary fire, either as a residuum from 

 the expiration of fusible and volatile materials, or more 

 generally as a deposit from volatile forms of matter/' This 

 statement is by no means liable to so many objections as 

 the first. During a visit which I made to Vesuvius during 

 the famous eruption of 1857, I was much struck with the 

 fact that no minerals could be obtained from the recently 

 ejected lava, while they could be obtained readily in the 

 more ancient ejections which form Monte Somma. This 

 gives great probability to the suggestion, that the minerals 

 found in lavas and other igneous products were due to a 

 residual, molecular, and segregatory action after the mass was 

 in fusion ; and the same evidence I saw in the recent lavas 

 of Iceland in my visit during last summer. But that even 

 the heat of a common fire was essential for the metamor- 

 phism, I cannot believe. You are all aware that Pompeii 

 was destroyed by hot ashes from Vesuvius, as so vividly de- 

 scribed by the younger Pliny ; and that Herculaneum was 

 overthrown by hot mud. Now, between the pillars of the stage 

 of the theatre at Herculaneum I extracted from the hardened 

 mud crystals of augite, which all the Plutonists affirm is of 

 igneous origin, but which must have been subsequently 

 formed after the deposit of the mud in the matrix, and at 

 natural temperatures, thus proving to me that augite may 

 be formed from volcanic eruptions after the liquid, not by 

 the fusible method, and that at a temperature not greater 

 than normally obtains. 



From these few and desultory remarks I have attempted 

 to indicate that mineralogy should take no unimportant place 

 in the curriculum of a geologist. Permit me now to give a 

 few general observations on the phenomena most interest- 

 ing to a mineralogist and geologist in a trip to Iceland, 

 more especially as they bear on our present subject. The 

 principal object I had in view in my journey to Faroe and 

 Iceland was to study the appearance of rocks of known 

 igneous origin, so as to compare them with those of our own 

 country, which are only believed to be of Plutonic origin. 

 It is very difficult for any geologist to look at any pheno- 

 menon without a leaning to some favourite theory. This I 



