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 difiicult for any geologist to look at any pheno- 
menon without a leaning to some favourite theory. This I 
