H. COATES AND P. MACNAIR ON BANDED HORNBLENDE SCHIST. 165 
Let US now see how far the theory advanced by Mr. Teall to 
account for the banded structure in the gneisses of the Lizard district 
is applicable to the phenomena seen at Balhoulan. In the first place, 
he shows that gneisses are on the whole identical with plutonic rocks 
in composition. The same position may, as we have seen, be taken 
with regard to the hornblende schists of Balhoulan, as they show 
strong affinities to many of the basic igneous rocks. It is to Mr. 
Teall’s second point we would, however, draw more particular atten¬ 
tion, namely, that plutonic rocks are often heterogeneous in structure, 
—that is, a great intrusion of plutonic rocks may often be seen to be 
composed of two or more distinctly different rocks. Typical examples 
of these “ plutonic complexes,” as they have been called, may be seen 
in our Perthshire Highlands, as at Tomnadashan, on the south side 
of Loch Tay, where a mica trap and a granite are involved, and in 
Glen Lednock, where a granite and a diorite are also seen intimately 
connected. Now, though at Balhoulan we have only one igneous rock 
in the form of a sill, yet the heterogeneous condition is supplied, as 
we have seen, by the existence of large masses of the clastic rocks 
which had been caught up and enveloped in the lava when it was in¬ 
truded into the latter in a molten state. These would be of all sizes, 
varying from the merest fragment up to large masses, and would, we 
think, be of the same value as a plutonic complex as regards 
supplying a heterogeneous mass for the deforming forces to operate 
upon. In the third place Mr. Teall, in his paper, proceeds to 
show how, given a heterogeneous plutonic mass, this mass may be 
deformed into a foliated and banded rock by the ordinary processes 
of regional metamorphism. Taking for granted that the two kinds of 
rock (the grit and the lava) were more or less thoroughly mixed when 
the great series of straining and shearing forces connected with the 
regional metamorphism of the Highlands first commenced, it would 
not be difficult to understand how the smaller fragments of grit would 
eventually be drawn out into the fine bands, while the more massive 
fragments would be represented by the thicker lenticular bands and 
masses of grit we have described. When the whole evidence is con¬ 
sidered, we think it is only upon such an hypothesis that the section 
seen at Balhoulan Quarry can be explained, and we hope that in 
bringing the section before the notice of the Society we have contri¬ 
buted a not unimportant illustration of Mr. Teall’s theory, and a 
striking example of the manner in which an originally crystalline 
igneous rock may be made to assume all the appearances of a clastic 
schist. We have also here one of those remarkable facts which are 
ever arising to confirm the great generalisation made by Hutton and 
Lyell many years ago, namely, that the forces at present operating 
upon the surface of the earth are sufficient to account for all the 
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