Sept. 9, 1886 ] 
NATURE 
449 
slowness. According to the same author a quartz grain 1/50inch 
in diameter requires to be transported by water action some 
3000 miles before losing its angles. On this account the presence 
in asandstone of numerous well-rounded grains is taken to indi- 
cate the action of wind, for, as is well known, blown sands are 
much more quickly rounded.? 
(7) Thus deposits of gravel and coarse sand, of considerable 
vertical thickness and great extension, are more likely to indicate 
the immediate action of a river than of a marine current. If 
limited in extent they may have been formed at the embouchure 
of ariver into a lake orsea. If, however, they can be traced 
for long distances, they are more probably in the main sub-aérial 
deposits from rivers. 
The following examples may convey some idea of the kind of 
river which would be required to transport the more important 
deposits of grits and stones mentioned in the first section of this 
address :— 
The old river-gravels of the Sierra Nevada are ‘‘in some 
places 300 or 400 feet thick and almost homogeneous from top 
to bottom,”’ sometimes they even obtain a thickness of 600 feet. 
Mr. Whitney is of opinion that the fall in these old river channels 
was probably from roo to 130 feet per mile. Apparently, how- 
ever, we need not invoke so large a fall as this. The total fall 
of the Danube is 3600 feet, and its length 1750 miles. From 
Passau to Vienna the fall is 1 in 2200, from Vienna to Old 
Moldava 1 in 10,000. Yet the velocity of the current from 
‘Vienna to Basias (fifteen miles above Old Moldava) is ‘‘ from 
two to three knots an hour,” depending on the amount of water. 
This would suffice to transport pebbles of the average size of the 
English Bunter. Below the Iron Gates the fall is still less rapid, 
but sand is carried down for a very considerable distance. If 
then the rivers of the old continental land resembled the larger 
streams of Europe, they would suffice for the transport of the 
materials with which we have dealt, especially if aided by coast 
currents after the debris had reached the sea. 
(8) If boulders occur in a matrix consisting of fine mud, or 
mainly of organic material, they must (unless they are volcanic 
bombs) have floated thither either attached to large seaweeds or 
entangled in the roots of trees, or supported by ice. If they are 
rather numerous and a foot or more in diameter, in a marine 
deposit, the last is the most probable mode of transport. A cubic 
yard of ice will more than suffice to float a cubic foot of average 
rock, 
Conclusion 
The facts already mentioned, regarded in the light of the 
above principles, justify, in my opinion, the following inferences 
as to the past physical geography of our country. At the com- 
mencement of the Cambrian period great masses of Archean 
rock, granites, gneisses, and schists, must have existed, not only 
on the western side of Britain, but also over a considerable tract 
of land now coyered by the sea. Detritus from this continent 
became an important constituent in the Cambrian rocks, and in 
many cases, as at St. David’s, in Anglesey, Carnarvonshire, &c., 
the shore-line must have been very near at hand. With the 
Cambrian period commences a long-continued subsidence, so 
that its basement beds at different places are very probably not 
all of quite the same age. The land surface was from the first 
irregular, and it is very probable that waves of the sea were 
fretting away some parts, while rain and river, heat and cold, 
were still sculpturing others. But among the materials of the 
ancient land were not only granitoid rocks, gneisses, and schists, 
but also newer strata more distinctly of clastic origin, memo- 
rials of past denudation—quartzites and grits, phyllites and 
slates, not to mention others—and these, by their intimate struc- 
ture, sometimes indicate that great earth-moyements must have 
already occurred.” In many localities, perhaps as a result of 
these disturbances, there occurred, towards the conclusion of the 
Archean period, great volcanic outbursts—by which, no doubt, 
numerous cones were built up, and many of the materials of the 
so-called Pebidian group were supplied. It is, I think, at present 
hardly safe to attempt to trace the exact land boundaries of 
the Cambrian ocean, but the enormous masses of Archzan 
material which are entombed in the earlier Paleozoic strata of 
Wales and of North-West Scotland can, I think, only be ex- 
plained by the proximity of a great continental land—an extension 
* See on the subject of this paragraph Daubrée, ‘‘ Géol. Expériment.,” 
S vol. i. sec. 2, ch. i., and J. A. Phillips, Q. 7. G. S., vol. xxxvii. p. 21, &e. 
AItis evident, for instance, that the north-west strike, and other effects of 
folding, had been produced in the Hebridean series of North-West Scotland 
before the Torridon sandstone was deposited. 
| 
of the present Scandinavian peninsula—which not improbably 
had a general slope towards the south-east, the main watershed 
of which may have lain some distance to the west of the Outer 
Hebrides.!. But even over the more central parts of Britain 
there cannot have been deep or open ocean, We are con- 
stantly coming upon the traces of pre-Cambrian and early 
Cambrian land; some of our Mid-England Archean masses, 
like the Malverns, appear to have risen above the water, and 
to have undergone denudation after the great earth-movements 
which ushered in the Silurian period. Prior to this, after a time 
of repose in the Cambrian, at more than one epoch, and in more 
than one place, there were great volcanic outbursts, which ap- 
pear to have studded the sea with volcanic islands, and to have 
added to the heterogeneous materials from which the sediments 
were now formed. It is evident that in Silurian times the coast- 
line had extended southward and eastward. The coarse deposits of 
this age, in Wales, the Lake district, and Southern Scotland, com- 
ared with the finer mudstones and limestones of the Welsh border 
and of England, seem fully to bear out this assumption, which is in 
accordance witha well-known law of mountain-making. The Old 
Red Sandstone of Scotland and of Wales indicates a yet further 
continental extension towards the south-east. A great epoch 
of mountain-making in the Scotch highlands, which had per- 
haps been going on at intervals from the beginning to the end of 
the Silurian period, had now come to an end ; the southern up- 
jands had risen up, like a Jura to the Alps. But probably 
their elevation did not terminate the earth-movements, for the 
post-Silurian cleavage of the Lake district, and the absence of 
Old Red Sandstone both here and in Central England indi- 
cate that the Palzeozoic land mass continued to extend on its 
south-eastern flank. The Devonian period introduces us in the 
greater part of Great Britain to an epoch of limited and excep- 
tional deposits, and of widely prevalent terrestrial conditions. 
It seems almost certain that the Old Red Sandstones of Scotland 
and Wales are of fresh-water origin—the deltas of rivers, formed 
either in lakes or possibly in part as sub-aérial deposits. Streams 
of considerable volume and of some strength are indicated by 
the materials. In one case, the Old Red Sandstone of North-East 
Scotland, we may perhaps discern in the Great Glen some indi- 
cation of the old river course. Itis easy to ascertain the source 
of the materials of the Scottish Old Red Sandstones. They are 
as obviously the detritus of the Highland mountains—then pro- 
bably a far grander and loftier chain—as the nagelflue and the 
molasse of Switzerland are of the Alps. 
At this time marine conditions prevailed in the south of Eng- 
land. The sea appears to have deepened towards the south, 
but I suspect that a region of crystalline rock still existed at no 
great distance in that direction and in the west. Probably the 
Brito-Scandinavian peninsula curved round to the east so as to 
include some part of Brittany.” Another great epoch of subsidence 
now commenced, commemorated by the formation of the Car- 
boniferous limestone. At this I need hardly glance, as it has 
been so fully discussed by Prof. Hull and other writers. The 
land sank both in the north and in the south of England. There 
was deep sea over Derbyshire and Southern Wales, but the 
ground beneath our feet probably remained above water, forming 
either a continental promontory or a large island. 
There were other well-known interruptions to the sea, which 
also overflowed a considerable part of Ireland and districts far to 
the east of England. The Scotch highlands, however, probably 
remained above water, for, as is well known, the Carboniferous 
limestone of Central Scotland overlies a fresh-water formation, 
and is itself not wholly marine, since it contains coal, and like 
conditions prevailed in Northumberland. 
Gradually, however, the sea shallowed, and terrestrial con- 
ditions returned. In the later part of the Carboniferous series 
we have clear indications of two, or perhaps three, important 
currents, almost certainly those of rivers, bringing materials, in 
the southern district from the west; in the northern, from the 
north-west and probably the north-east. These materials may 
have been in part derived from older Palaeozoic rocks, but the 
facts when fairly considered seem to indicate that there was also 
an extensive denudation of crystalline and not improbably 
Archzan rocks, unless we suppose that great areas of eruptive 
Palzeozoic granite have now disappeared beneath the waters. At 
any rate, we may perhaps regard the open water between Ireland 
1 Possibly the comparatively rapid deepening of the Atlantic beyond the 
too-fathom line may have some relation to the western outline of this 
primeval Atlantis. _ ys 
2 Compare, as an illustration, 
western side of Italy. 
the curving round of the Alpine chain on the 
