104 Messrs. Hicks and Davies— 
of the Hebrides. In a general way this may be correct; but as it is 
now granted by many that these rocks are metamorphosed sediments, 
a detailed description of all the varieties noted in each separate 
area seems to be the only true method by which a correlation can 
be attempted. Evidence of age also in proportion to their crystalline 
condition, and the mineral change which has taken place, is frequently 
a still better guide to correlation, and should be carefully noted in 
all cases. 
The physical evidence of having been subjected to contemporaneous 
movements, as indicated by a prevailing strike in the beds, as insisted 
upon by Murchison, is undoubtedly also of immense importance, and 
should never be lost sight of in these inquiries. 
In the paper already referred to I mentioned that some of the so- 
called eastern gneiss rocks of Central Ross-shire about Ben Fyn, 
etc., were almost identical in character with some of the western 
gneiss which I had examined about Gaerloch. It is well known 
that Murchison, Geikie, and others maintained that there is a great, 
dissimilarity between the two groups everywhere. Physically and 
mineralogically they are utterly unlike according to these authors. 
Nicol, and before him, Cunningham and Macculloch, considered 
them on the whole to be more or less identical, or at least sufficiently 
so to be associated together in one great group. Though differing 
from the last-named authors on some points which will be specially 
referred to again, I yet agree with them that the eastern and western 
series (excluding some beds which I consider to belong to the un- 
altered Cambrian and Silurian sediments) are sufficiently identical 
in their mineral characters, in their crystalline conditions, and in 
their physical aspect generally, to belong to the same great group of 
metamorphosed sediments of Pre-Cambrian age ; necessarily unlike 
in some respects, at various horizons, but showing everywhere 
characters special enough to prove that they have been equally sub- 
jected to the same influences throughout, whether chemical or 
physical. 
_ To prove this it will be necessary to examine carefully into the 
completeness or otherwise of the changes which have taken place in 
all the types of rocks found in each area, and whether there has 
been throughout an equal change of constitution in those which 
appear to be sufficiently identical for this comparison to be made. If 
we find that these rocks are equally metamorphosed, and that there 
is an equal amount of various crystalline minerals disseminated 
throughout which could not have been present, in that state, in the 
original sediments, then we may fairly claim that the rocks have 
undergone an equal change of constitution: and therefore that they 
must have been subjected to the same long-continued influences, 
and are geologically speaking of the same age. 
The strongest advocates of the views held by Murchison, that the 
eastern types of gneisses, hornblende rocks, mica schists with garnets, 
ete. (true crystalline schists found in many parts of Central Suther- 
land and Ross and other parts of the Highlands), are younger than 
the Cambrian, and of some of the Silurian rocks, have in no case, as 
