24 
PROCEEDINGS OE SOCIETIES. 
and sands were being deposited that 
we now recognise as fine-grained 
slates and gritstones. That from these 
active volcanic vents, flows of molten 
rock were taking place, some of which, 
being more purely felspathic, consoli¬ 
dated into felstones; others, having 
the materials for the formation of 
hornblende mingled with the felspa¬ 
thic base, produced greenstones. Both 
kinds were accompanied by mecha¬ 
nically formed ashes and conglome¬ 
rates, just as the trachytic and dole- 
ritic lavas of the present day are 
accompanied by tuff and peperino, 
and volcanic breccias. It follows, of 
necessity, that these contemporaneous 
sheets of trap had their origin some¬ 
where in intrusive pipes, veins, and 
larger masses, proceeding from the 
interior towards the surface; and it is 
probable that the contemporaneous 
sheets of greenstone proceeded from 
masses of intrusive greenstone, and 
that the felstones proceeded from 
dykes and veins of crystalline fel- 
stone; in other words, from a crystal¬ 
line aggregate of quartz and feldspar, 
which is, in fact, that variety of gra¬ 
nitic rock which I have called Elva- 
nite, and which, whenever it contains 
a micaceous mineral as a constituent, 
becomes true Granite. 
We must, therefore, look to granite 
veins and intrusive masses as the deep- 
seated portion of that mass which, 
when poured out at the surface, be¬ 
comes felstone—a rock which has the 
same feldspar and quartz in a pasty 
condition, that in the Elvanite are 
crystallized out. We may then fairly 
suspect that many of the numerous 
elvan dykes and veins, and some of 
the granite masses must closely agree¬ 
ing with Elvanite, were in fact the 
deep-seated roots, as it were, from 
which the felstones have proceeded. 
Although, then, these dykes and 
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Scale, both vertical and horizontal, 6 inches to 1 mile. 
