26 
Tvliich attaches to the formation of silica is especially in relation to 
the origin of numerous so-called igneous rocks, of Trhich granite 
may he taken as the t 3 rpe. It is Trell known that the theory of 
Werner on the Ne]3tunian origin of granite was afterwards aban¬ 
doned by geologists, and replaced by the Plutonic theory, and for a 
long time it has been the received opinion that all granite has been 
the result of rock fusion at a very high temperature. But those 
persons who have been accustomed to make experiments on the 
fusion of mineral substances at high temperatures have been 
familiar with certain difficidties which stood in the way of accepting 
this view of the formation of the so-called igneous rocks. Dr. 
Procter then considered the formation of the other constituents, and 
showed that felspar and mica could be formed by both the diy and 
wet method, but more readily by the latter. In exaniining the 
structure of granite, it is always found that quartz, the least fusible 
of the three constituent minerals, appears to have been the last to 
crystallize, whilst felspar, the least fusible of them, appears to have 
crystallized first; the felspathic crystals being imbedded in those of 
mica and quartz. Hence it would appear that the constituent 
minerals of these rocks are not crystallized in such an order of 
succession as would be indicated by their relative degree of fusibil¬ 
ity, and as might be expected if they were of igneous origin. It 
was then shown that the doctrine of superfusion was insufficient to 
account for the phenomena, besides certain chemical reasons which 
rendered such explanation highly improbable. Other arguments 
were drawn from the presence of certain minerals in granite, 
such as allanite, gadolinite, which become altered by the 
action of heat in their chemical and physical characters. It was 
then shown that lava, the well-known result of fusion, rarely, if 
ever, in the recently ejected strata, contained crystals of any size. 
Such are most frequently found in old lavas, and are formed by 
accretion, ffiom the water permeating and taking up the necessaiy 
ingredients to produce the special crystal by deposition from the 
solution, in the same manner that crystals of calc-spar have origin¬ 
ated from coralline limestones. Hence there is no real analogy 
between the crystalline granite and lava. An extract was then read 
from a paper by Mr. Sorby, in which he concludes that granite has 
originated from aqueous solution under great pressure. In con¬ 
clusion, it was said that it can scarcely be conceived that the 
elements of granite have been in a complete state of watery solution, 
from which by degrees they have been separated by crystallization. 
