“ts 
Sept. 9, 1886] 
NATURE 
445 
some pebbles of vein-quartz and two or three beds of quartzite. 
The grains in these appear to have been derived from old granitoid 
rocks, and not from the porphyritic rhyolites of the district. In 
one case, at the Brande, they are most conspicuously rolled, and 
this has happened, though less uniformly, in a grit from near the 
ruins, Bradgate, which also contains grains of compound structure. 
In conclusion, I must briefly notice the so-called Torridon sand- 
stone of North-Western Scotland, which is in many respects 
invaluable to the student. That it is not later than the base of 
the Ordovician is indisputable ; that it is underlain by and derived 
from a mass of Archzean rocks—gneisses, more or less granitoid, 
with occasional schists—is universally admitted. Its coarser base- 
ment beds are crowded. with fragments of the underlying gneisses 
and schists, and since the epoch of their formation no material 
change has taken place in either the one or the other. The finer 
beds, though other materials occasionally occur, are largely, 
sometimes almost exclusively, composed of grains of quartz and 
of feldspar identical in every respect with those in the underlying 
series. It may be a fact of some significance, for it agrees with 
what I haye elsewhere noticed in very old fragmental rocks, that 
the feldspar appears to have been broken off from the parent rock 
while still undecomposed, and in many cases is even now remark- 
ably well preserved. It would seem, therefore, as if the denu- 
dation of the granitoid rock had been accomplished without 
material decomposition of its feldspar, but I must not allow myself 
to digress into speculations on this interesting and suggestive 
fact. While referring to this district I may mention the quartzites, 
though, strictly speaking, they are Ordovicianin age. These in 
some cases consist all but exclusively of quartz grains derived 
from the Archzean series, which, however, are generally smaller 
than those in the Torridon ; it would seem as if the feldspar of 
the parent rock had either decomposed z# sit, or had been 
broken up in consequence of the longer distance from the source 
of supply. This quartzite is sometimes of singular purity, con- 
taining little or no earthy material, and only rarely a flake of 
- mica or a grain of feldspar, tourmaline, or epidote (?). 
Ordovician- Silurian.—In regard to the earlier of these forma- 
tions I am better acquainted with the volcanic than with the 
non-volcanic fragmental beds. Still, so far as I have seen, we 
find among the latter frequent indications of a supply of materials 
from regions of crystalline as well as of ordinary sedimentary 
rocks. The quartzite of the Stiper Stones (possibly earlier than 
the Arenig) appears to have derived most of its grains from 
granitoid rocks, and probably the same is true of many of the 
coarser beds in the Caradoc group of Shropshire and Eastern 
Wales. The Garth grit of Portmadoc appears to have derived 
much of its quartz from a like source as the Stiper Stones, but 
it also contains bits of a fine-grained quartzose schist and of 
older clastic rocks. A grit from the Borrowdale series of 
Chapel-le-dale contains, in addition, bits of old andesite and 
probably diabase, with fragments of a rather granitoid gneiss 
and quartzose schists. Fragments of cry-talline rock, both 
small and large, abound in the Upper Llandovery beds at 
Howler’s Heath, at Ankerdine Hill, in the Abberley district, on 
the west flank of the Malverns, and at May Hill, thus indicating 
that early in Silurian times far greater outcrops of crystalline 
rock existed than are now visible west of the Severn. Mr. W. 
Keeping (Quart. Journ. Geol. Soc. vol. xxxvii. p. 149, &c.( calls 
attention to the abundance of fragments of quartz, feldspar, and 
mica in the ‘‘ greywackes” of the Aberystwith district, which 
give the rock sometimes quite a granitoid appearance, and adds 
that, in his opinion (2dzd. p. 150), ‘‘ the abundance of feldspar 
erystals, so general in the Silurian rocks (Upper Silurian of 
North Wales, South Wales, and the Lake district), points to the 
_ neighbouring presence-of a vast mass of early, perhaps primeval, 
igneous rocks as the great source of sediment supply in Silurian 
times.” What I have seen of the Denbigh grit of North Wales 
and of the Coniston beds of the Lake district confirms this con- 
clusion. It is true that some of the material may have been 
supplied by Ordivician volcanic rocks, and that the quartz grains 
in the specimens which I have examined are not large. But 
we must remember that the latter can hardly have been furnished 
by the lavas of the Lake district ; and those of North Wales, 
though richer in silica, do not, so far as I know, generally con- 
tain large quartzes. These, indeed, may have been derived 
from the denudation of Cambrian rocks, but I should doubt 
the sufficiency of such an explanation. In one specimen, a Den- 
bigh grit from Pen-y-glog, near Corwen, there occurs, besides 
one of smaller size, a fragment about ‘1 in diameter, exhibiting a 
micrographic arrangement of quartz and feldspar. In Cornwall, 
among beds which are almost certainly Ordovician or Silu- 
rian, we find similar evidence of derivation from much more 
ancient rocks. The conglomerates of the Meneage district con- 
tain, in addition to quartzites, greywackes, and other old sedi- 
mentary beds, abundant fragments of a moderately coarse- 
grained granitoid rock, and occasionally a hornblendic rock 
similar to the well-known Lizard schist. A series of specimens 
which I have examined microscopically shows, in addition to 
compact igneous rocks, apparently volcanic, quartz grains pro- 
bably derived from granitoid rock, various fine-grained schists 
and schistose argillites or phyllites, quartzites, grits, and other 
older clasticrocks. One fragment of schist contains little eyes of 
feldspar, and in general structure reminds me of some in the so- 
called ‘‘Upper Gneiss” series of North-Western Scotland. 
Another, a fine-grained mica-schist or a phyllite, exhibits a cleay- 
age transverse to the rumpled foliation. 
A rich harvest probably awaits the explorer in the “‘ grey- 
wackes” of the southern uplands of Scotland. A ‘* Lower 
Silurian” conglomerate from Kingside, Peebles-shire, con‘ains 
numerous fragments of igneous rocks, probably of volcanic 
origin, and bits of granitoid rock, with some which are either 
very old quartzites or perhaps vein-quartz. These have been 
crushed and re-cemented before being detached from the parent 
rock. The basement conglomerate of the Craig Head limestone 
group (Llandeilo-Bala) is full of rounded fragments of volcanic 
rocks. These, as in the last-named case, exhibit considerable 
variation ; the majority, however, are probably andesites, and 
perhaps in one or two cases even basalts. A Middle Llandovery 
conglomerate from near Girvan is largely made up of fragments 
which appear to"have been derived from very ancient quartzose 
rocks. The greywacke of rather later age from near Heriot, 
Edinburghshire, contains, with numerous volcanic fragments, and 
a little argillite, a few bits of fine-grained quartz-schist, together 
with grains of quartz and feldspar, suggestive of derivation from a 
more coarsely crystalline rock. 
Old Red Sandstone and Devonian.—lt is, 1 believe, indisput- 
able that when the Old Red Sandstone of Scotland was formed 
a great period of mountain-making had ended and one of 
mountain-sculpture was far advanced. The conglomerates are 
often full of fragments of the crystalline rocks of the Highlands, 
and no doubt the sandstones derived their quartz grains from the 
same source. In the southern half of the country, however, as 
is well known, volcanic materials, more or less contemporaneous, 
play an important part. I have not been able to examine closely 
the Old Red Sandstones of England and Wales, but their fre- 
quent near resemblance to the sandstones of Scotland suggests 
a similar derivation. True, the materials may have been sifted 
from older clastic rocks, but there is nothing specially to suggest 
this, and the abundant pebbles of vein-quartz, which I have seen 
in one or two localities, seem rather more favourable to the other 
hypothesis. I have only examined microscopically a very few 
specimens of Devonian grit, all from the south side of the country. 
These certainly seem to have derived their materials, in part at 
least, from crystalline rocks, both granitoid and schists of finer 
grain; one specimen also apparently containing some bits of 
hypometamorphic rock. 
Carboniferous.—In Scotland some of the basement beds of 
this series so closely resemble the Old Red Sandstone that no 
further description is needed, and the same remark may be made 
of the very few overlying sandstones which I have carefully ex- 
amined. In the North of England the basement conglomerates, 
so far as I have seen them, are made up of earlier Palaeozoic 
rocks, but for many of the great masses of sandstone which occur 
in the series a source of supply is not so easily found. Dr. Sorby, 
who has made a special study of the Millstone grit of South 
Yorkshire, tells us that it is formed of grains of quartz and 
feldspar, apparently derived froma granite, and contains pebbles, 
sometimes an inch or so in diameter, of vein-quartz, of hard grits, 
of an almost black quartz-rock or quartz-schist, and of a non- 
micaceous granite. He also notes one fragment of a greenstone, 
and another either of a fine-grained mica-schist or of a clay-slate. 
The granite, he states, more resembled those of Scandinavia 
than any one now visible in Britain, and the bedding indicated 
a supply of materials from the north-east. In the Millstone grit 
near Sheffield he says that the grains appear to be but little 
worn, as if they had not been drifted from far. A few also appear 
to have been derived ftom schists. From what I have myself 
seen, I anticipate that Dr. Sorby’s conclusions may be extended 
to most of the other coarser Carboniferous clastic beds of Northern 
England, except that, perhaps, as was inferred by Prof. Hull, 
