186 
IDDINGS. 
present had been the same, and that they have had the same 
geographical distribution. This seems to the writer to have 
been the case with the great succession of volcanic eruptions 
that have occurred in Great Britain. But the recurrence of 
chemically different groups may be susceptible of another 
interpretation, which has been pointed out to the writer by 
Mr. Clarence King, who considers that the chemical charac¬ 
ter of the undifferentiated localized magma will depend on 
its position in depth within the earth, and that the depth at 
which solid matter is converted into fluid matter will be 
controlled by the loading or unloading that takes place at 
the surface. This hypothesis may be tested by comparing 
the character of each general magma with the geological 
history of the region in which it occurs. 
In either case the geographical distribution of chemically 
different groups of igneous rocks proves that they have origi¬ 
nated immediately from localized magmas, and not from a 
liquid zone of homogeneous magma. 
RESUME. 
In conclusion, it may he well to trace the development of 
ideas regarding the nature and origin of igneous rocks as 
they have been expressed by the investigators cited in the 
historical portion of this paper, and to point out in what 
respect the views of the present writer tend to modify them. 
The conception of a differentiation of a primary homoge¬ 
neous mass as the cause of the differences in volcanic lavas 
was expressed definitely by Scrope in 1825. To his mind 
the previously undifferentiated mass was a crystalline rock 
with the character of granite, and the process by which the 
separation was supposed to have taken place was described 
as an intumescence due to interstitial water being highly 
heated and dissolving part of the quartz in the rock, and 
which was accompanied by the volatilization of the mica 
and by the forcing out of the gaseous portion by pressure, 
and its concentration and crystallization elsewhere. 
