156 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
In a paper contributed to the Geological Magazine for the 
year 1877* by Mr. J. J. Harris Teall, on the origin of certain 
banded gneisses, the author advances a hypothesis to show how, 
under certain conditions, a crystalline igneous rock may eventually, by 
the ordinary processes of dynamic metamorphism, assume a perfectly 
banded structure. In the course of his paper he shows the a priori 
probability of his theory by an experimental illustration with coloured 
clays; showing that if clays of two colours be intimately mixed and 
then drawn out to represent the shearing process they will eventually 
assume much of the appearance presented by the banded hornblende 
schists of the Lizard. He next goes on to observe (i) that banded 
gneisses are on the whole identical with plutonic rocks in composition; 
(2) that masses of plutonic rocks are often heterogeneous; and (3) that 
heterogeneous masses, if such exist, may be deformed in the manner 
required to produce banded structures. He then discusses the igneous 
rocks of the Lizard and their sheared representatives. In conclusion, 
and with reference to the granular structure of these rocks, he says:— 
“ It is undoubtedly true, as Professor Bonney has pointed out, that 
many of the rocks are largely composed of broken crystals, and may 
be said therefore to possess a clastic structure, if we use the term 
clastic in its etymological sense. But this is no proof that the fragments 
have been deposited as such. The original minerals may have been 
broken during the deformation of the rock masses. This, I believe, 
is what has actually taken place. The structures are of the kind 
for which Professor Kjerulf has proposed the term ‘ cataclastic 
In a paper read before the Geological Society of London,! 
Major-General C. A. M‘Mahon gives an account of the Hornblende 
schists of the Lizard district, and advances a theory to explain their 
origin. He also criticises the theory advanced by Mr. Teall as 
already cited, and opposes the view that such rocks as the banded 
hornblende schists of the Lizard could be produced by shearing 
movements. He maintains that, granted the volcanic origin of the 
schists, he has not been able to find any evidence showing that 
dynamic metamorphism has at all effected these schists, nor can he 
see how their banded structure could be produced by such move¬ 
ments. He says that not only would the regular succession and 
alternation of these thin bands present a serious objection to the 
acceptance of this explanation, but it is obvious that the shearing of 
a solid rock into such extremely thin layers would have developed 
heat sufficient to fuse the whole mass, in which case it would have 
lost its banded structure and assumed that of a hornblende granite. 
The stripes are so sharply defined and thin that several of them can 
* Geological Magazine, 1877, P- 4 ^ 4 - 
\ Qiiar. Jour. Geol. Soc., Vol. XLV., 1889., p. 519. 
