Sept. 9, 1886 | 
NATURE 
443 
of the size of the crystalline constituents of ordinary granites, 
and the more coarsely crystalline gneisses. But speaking of those 
which enter into the composition of the ground mass, I should 
say that the individual quartz grains do not often exceed ‘03”, 
and are very frequently between this and ‘02’. In the finer- 
grained granites and more distinctly banded gneisses, and their 
associated quartziferous schists, about ‘or’ is a common size, 
while in the finer schists (believed by many geologists to be later 
in date than the aforesaid) they range from ‘oo2” downwards, 
and do not generally exceed ‘oor’. Feldspar crystals, where they 
occur, probably do not differ very materially in area from the 
quartz, though they are often, as might be expected, rather 
longer and narrower ; mica crystals, cut transversely, are often 
longer and usually much narrower. Of other constituents, as 
being either rarer or more liable to change, I will not speak in 
detail. The individual quartz grains, in the compact and glassy 
varieties of the more acid igneous rocks, are about the same 
size as those in an ordinary granite. 
Space does not permit me to enter upon the methods of dis- 
tinguishing between the materials furnished by the different 
varieties of crystalline schists, gneisses, and igneous rocks of 
similar chemical composition. For the most important of these 
I must refer you to Dr. Sorby’s address, but I may add that there 
are others which it would be almost impossible to describe in 
words, as they can only be learnt by long-continued work and 
varied experience. I do not pretend to say that in the case of a 
grit composed of fragments of about ‘o2” diameter we can suc- 
ceed in identifying the parent rock of each individual, but I 
believe we can attain to areasonable certainty as to whether any 
large number of its constituents have been furnished by granitoid 
rocks, by banded gneisses and schists, by fine-grained schists or 
certain phyllites, by older grits or argillites, or by lavas and 
scoria. There seem to be certain minute differences between 
the feldspars from a granitoid rock and from a porphyritic 
lava, and more markedly between the quartz grains from the 
tworocks. The latter can generally be distinguished from the 
polysynthetic grains furnished by certain schists or veins, and 
these not seldom one from another. Obviously the larger the 
fragments the less, ceterts paribus, the difficulty in their identifi- 
cation. When they exceed one-tenth of an inch the risk of 
important error is, I believe, to a practised observer compara- 
tively small. 
Obviously, also, the shape of the grains leads to certain 
inferences as to the distance which they have travelled from their 
original source, and as to the means of transport, but into the 
details of this I must forbear to enter. I will merely remind 
you that small angular fragments of quartz are so slowly 
rounded when transported by running water that, if well- 
rounded grains appear in large numbers in a sandstone, it seems 
reasonable to suppose that these are, in the main, wind-drifted 
materials, 
Thus every rock in which the constituent particles admit of 
recognition and of identification may be made to bear its part in 
the work of deciphering the past history of the globe. Where 
the constituents have been derived from other rocks, we obtain 
some clue to the nature of the earth’s crust at that epoch ; where 
the locality whence a fragment was broken can be discovered, 
the nature, strength, and direction of the agents of transport can 
be inferred. Some idea as to the structure and surface-contour 
of the earth in that district, and at that time, can be formed ; 
and thus the petrologist, by patient and cautious induction, may, 
in process of time, build up from these scattered fragments the 
long-vanished features of the prehistoric earth, with a certainty 
hardly less than that ofthe palzeontologist, when he bids the dry 
bones live, and repeoples land and sea with long-vanished races. 
The latter study is in vigorous maturity, the former is still in its 
infancy : so much wider then is the field, so much more fasci- 
nating, to many minds, is the investigation. There are many 
districts which are without fruit for the palzeontologist—there 
are few indeed which, to the petrologist, do not offer some hope 
of reward. The field of research is so wide that not one nor few 
men can gather all its fruits. It needs many workers, and it is 
in the hope of enlisting more that I have ventured to bring the 
subject before you to-day. 
Materials of the Coarser Fragmental Rocks of Great Britain » 
I proceed now to give a brief epitome of the constitution, so far 
* I have been obliged to exclude those of Treland, as I have so little 
material from that country, and for want of space h i 
Soe aera Pp ave not dealt fully with 
as I know it, of our British grits, sandstones, breccias, and con- 
glomerates. I shall exclude, as involving too many collateral 
issues, the Post-Pliocene beds, and dwell more on the earlier 
than on the later deposits, because the latter obviously may be 
derived from the former by denudation, so that it becomes the 
more difficult to conjecture the immediate source of the con- 
stituent particles. Further, in order to avoid controversy on 
certain questions of classification, or for brevity, I shall occasion- 
ally group together geological formations which I think separable. 
It may be convenient, however, to call your attention to the 
localities at which, at the present day, granitoid rocks (many of 
which may be of igneous origin, but are of very ancient date), 
gneisses, and crystalline schists are exposed in Great Britain, 
as well as those where considerable masses of igneous rock of 
age not later than Mesozoic occur. The former constitute a 
large part of the north-western and central highlands of Scotland 
and of the islands off its west coast ; they are exposed in Angle- 
sey and in the west and the north of Carnarvonshire ; they form 
the greater part of the Malvern Chain, and crop out at the 
Wrekin ; they occur on the south coast, at the Lizard, and in 
the district about Start Point and Bolt Head ; they rise above 
the sea at the Eddystone. It is probable that these last are the 
relics of a great mass of crystalline rock, which may have ex- 
tended over the Channel Isles to Brittany ; also, that we may 
link with the massif of the Scotch highlands the crystalline rocks 
of Western Ireland on the one hand, and of Scandinavia on 
the other. Among the indubitably igneous rocks we have 
granite, or rocks nearly allied to it, im Scotland, in the 
Lake district, in Leicestershire, and in Devon and Corn- 
wall. Feldstones, old lavas, and tuffs of a more or less acid 
type occur in Southern Scotland, to some amount also in the 
Highlands, in the Lake district, and in various localities of 
rather limited extent in West-Central England, as well as 
in the south-west region just mentioned, while in Wales we 
have, in the northern half, distinct evidence of three 
great epochs of volcanic outbursts, viz. in the Bala, in the 
Arenig, and anterior to the Cambrian! grits and slates. In 
South Wales there were great eruptions at the last-named epoch 
and in Ordovician times. I have passed over sundry smaller 
outbreaks and all the more basic rocks as less immediately con- 
nected with my present purpose. It is, I suppose, needless to 
observe that a coarsely crystalline rock, whether igneous or of 
metamorphic origin, must be considerably older than one in 
which its fragments occur. 
Cambrian and later Pre-Cambrian.—That the majority at 
least of the gneisses and crystalline schists in Britain are much 
older than the Cambrian period will now, I think, hardly be 
disputed by any who have studied the subject seriously and with- 
out prejudice. There are, however, later than these, numerous de- 
posits, frequently of volcanic origin, whose relation to strata 
indubitably of Cambrian age is still a matter of some dispute. 
Therefore, in order to avoid losing time over discussions as to the 
precise position of certain of these deposits, or the particular 
bed which in some districts should be adopted as the base of the 
Cambrian, I will associate for my present purpose all the strata 
which, if not Cambrian, are somewhat older. The latter, how- 
ever, exhibit only micro-mineralogical changes, and of their 
origin, volcanic or clastic of some kind, there can be no reason- 
able doubt ; so that the difference in age does not appear to 
be enormous ; that is to say, I include with the Cambrian the 
Pebidian of some recent authors, 
The utility of microscopic research has nowhere been 
better exemplified than in the case of the oldest rocks of St. 
David’s. Some authors have supposed that the base of the 
Cambrian series in this district has been ‘‘ translated” beyond 
recognition, others that it has been thrust out of sight by the 
intrusion of granitic rock. But low down in the series, beneath 
the earliest beds that have as yet furnished fossils to British 
palzontologists, there is a well-marked and widespread conglo- 
merate ; underlying this, with apparent unconformity, comes a 
series of beds very different in aspect, chiefly volcanic, and 
beneath this a granitoid rock. The conglomerate, in places, 
even without microscopic examination, proves the existence, 
though probably at some distance, of more ancient rock, as it is 
full of pebbles of vein-quartz and quartzite ; but in other parts 
it is crowded with pebbles closely resembling the feldstones in 
the underlying volcanic group, and in some parts becomes a 
regular avxose, made up almost wholly of quartz and feldspar, 
* I take the base of the Arenig as the commencement of the next forma- 
tion, the Ordovician, which thus represents one phase of the Lower Silurian 
in the variable nomenclature of the Geological Survey. 
