448 
Somersetshire, have been so fully worked out by Mr. Etheridge 
and Mr. Ussher as to require but a passing notice. It is evident 
that they differ somewhat in date, though probably all may be 
referred to the age of the Keuper, and that they are local breccias 
or conglomerates formed around the margin of islands or on a 
continental coast-line during a gradual subsidence and in com- 
paratively quiet waters. 
Jurassic.—Coarse detrital material is not very common in the 
Jurassic series. The limited Rhetic beds indicate a transition 
from the peculiar physical conditions of the Keuper to the marine 
conditions of the Lias, and the sediment in them was probably 
derived from the same source as the Keuper marls. Great clay 
beds also occur, as is well known, throughout the Jurassic series ; 
and the sandstones, so far as I have been able to examine them, 
do not enable me to offer any sugzestions as to their origin. Pro- 
bably some of the grains were originally derived from granitoid 
rocks, but they may have been directly obtained from other 
sandstones. <A grit, however, in the estuarine series of the lower 
oolites of Yorkshire (Mr. Phillips’s collection) looks as if it 
might. have been partly derived from a schist, but as this is the 
only rock from the northern area which I have had the 
opportunity of minutely examininz, it would be imprudent to 
speculate. 
Neocomian-Cretaceous.—I have examined very few specimens 
from the fresh-water Neocomians of the south of England, but, 
so far as I have seen, I should think it probable that the quartz 
had been derived from old crystalline rocks, though perhaps not 
immediately. The same remark applies to the sands of the 
upper and marine series, which, in one instance at least, exhibit 
exceptionally rounded contours.! Among these, however, con- 
glomeratic beds occur which have already attracted some 
attention. It is obvious that no small part of the materials, as 
at Farringdon, Potton, and Upware, has been derived from 
fossiliferous secondary rocks of earlier date. There are also 
pebbles of vein-quartz and quartzite which, however, may have 
heen obtained by the denudation of ‘Triassic rocks. The 
‘Lydian stone,” which is abundant in angular or subangular 
fragments at Potton and Upware, is for the most part chert 
from the Carboniferous Limestone, or in some cases from Jurassic 
rocks, but a few specimens may be flinty argillites, and thus of 
greater antiquity. One or two pebbles of older Palzeozoiec rock 
have been found, and a hard quartz grit has occurred, containing 
among its grains minute acicular crystals, probably of tourmaline. 
Potton has furnished one or two pebbles which appear to be a 
devitrified pitchstone, and a large pebble of porphyritic quartz- 
felsite has been sent to me by Mr. Willet from Henfield (Sussex). 
These conglomerates, together with others in the Upper Neo- 
comian of England, have been so fully described by Mr. Walter 
Keeping (Geol. Mag. Dec. 2, vol. vii. p. 414), that I need not 
enter into further details, though I am well aware that the subject 
is by no means exhausted. 
For a like reason I may pass briefly over the remarkable 
erratics found in the Cambridge greensand (Sollas and Jukes- 
Browne, Q. ¥. G. S. vol. xxix. p. 11). They occasionally 
slightly exceed a cubic foot in volume, but are generally smaller. 
Among them are diverse sandstones and grits, probably Palzo- 
zoic, granite, gneiss, various schists, quartzites, and slates, besides 
greenstone, a very coarse gabbro or hypersthenite, and a com- 
pact feldstone. I think it highly probable that many of these 
erratics came from the north, in some cases almost certainly from 
Scotland, and were transported by ice, though I am not satisfied 
that any exhibit true glacial striz. In the south of England a 
boulder of old quartzose rock, perhaps a piece of a coarse quartz- 
vein, crushed and re-cemented, has been found by Mr. J. S. 
Gardner in the gault, and in the chalk we have the well-known 
cases of the granitic rock and other boulders at Penley, near 
Croydon, and of coal (Wealden or Jurassic) in Kent (Godwin- 
Austen, 0. ¥. G. S. vol. xvi. p. 327). Mr. Godwin-Austen 
describes other instances of pebbles in chalk, and I have received 
two or three small specimens from Mr. W. Hill, from about the 
horizon of the Melbourne rock, which, however, have not yet 
thrown any light on the subject. 
Locene.—Previous writers have called attention to the fact that 
the sand of the Thanet, Oldhaven, and Bagshot beds is mainly 
composed of quartz. This is abundantly confirmed by my own 
observations. So far as I have seen, in all these the grains are 
not, as a rule, conspicuously rounded. It can hardly be doubted 
* Prof. Rupert Jones has called attention to sand-worn pebbles in the 
Upper Tunbridge-Wells sandstone of the Weald (Geo/. Mag. Dec. 2, vol. v. 
p. 287). 
INA TORE 
[ Sept. 9, 1886 
that older sandstones or granitoid rocks lying to the west have 
furnished the materials of the Bagshot series, which still has so 
wide an extension in that direction ; their lithological similarity 
would lead us to took towards the same quarter for the materials 
of the more limited Oldhaven and Thanet beds. The well-rolled 
flint pebbles in the Oldhaven series, and in occasional layers in 
the Bagshot, suggest the proximity of a shore-line of Upper 
Cretaceous rocks. 
I have had no opportunity of adding to what has been written 
on the lithology ofthe limited Pliocene deposits in England, and, 
as stated at the outset, have excluded from the scope of this 
address all beds of later date. which have been so ably discussed 
by Mr. Mackintosh, Dr, Crosskey, and many other geologists. _ 
Principles of Interpretation 
In attempting to interpret the facts which I have enumerated 
we must bear in mind the following principles :— 
(1) Pebbles indicate the action either of waves of the sea,! or 
of strong currents, marine or fluviatile. 
(2) The zone in the sea over which the manufacture of peb’ Jes 
can be carried on is generally a very narrow one. It extends 
from the high-tide line to the depth usually of a few feet below 
low-water mark. It is estimated that as a rule there is no 
disturbance of shingle at a greater depth than twenty feet below 
the latter. It is therefore probable that a thick and very widely 
extended pebble bed is not the result of wave action. 4 
(3) The movement of the deeper waters of the sea asa rule 
is so slight that only the very finest sediment can be affected by 
it. Now and then great currents like the Gulf Stream, or more 
locally ‘‘races,”” may have sufficient power to transfer pebbles 
and sand, but instances of this will be exceptional, and confined 
to rather shallow water. The larger coast currents, however, 
may transport mud to considerable distances, but in directions 
parallel with the main trend of the shores. 3 
(4) Except where very large rivers discharge their water into 
the ocean, or in sorne special case of (3), sediment is deposited 
comparatively near the shores of continents. Even in the case 
of very large rivers only the finer sediment is carried far from 
land. The Challenger soundings have shown that 150 miles is 
about the maximum distance from land within which any im- 
portant quantity of detrital materials is deposited.? As a rule (so 
far as I can ascertain), the coarser sediments are generally 
deposited within a few miles of the coast. Hence this is fringed 
by a zone of sediment, which, after passing a maximum thickness 
within a short distance from the shore, gradually thins away. I 
doubt whether this detrital fringe is often more than seventy or 
eighty miles wide ; probably the coarser sands do not usually 
extend for so much as a quarter of this distance. The inertia of 
the mass of the ocean water quickly arrests the flow of even the 
mightiest river or reduces it to a mere superficial current. Hence 
the great ocean basins are regions where rock-building is carried 
on slowly and chiefly by organic agency. ‘Their borders bear 
the burden, and the load taken off the continent is laid down 
on the bed of the adjacent sea. 
(5) Thus rain and rivers are generally more important agents 
of denudation and transportation than the sea, because unless 
the land be rising or sinking the zone over which the latter can 
operate is limited both vertically and horizontally. 
(6) The coarser materials of rock are capable of being trans- 
ported by streams to considerable distances without serious 
diminution of volume. Prof. Daubrée has proved experimentally 
that a stream flowing at the rate of about two miles per hour 
would roll angular fragments of quartz or hard granite into 
perfectly smooth pebbles after a transit of 25 kilometres 
(15% miles). During this process the fragments lost about four. 
tenths of their weight. Further transport reduced the volume of 
the pebbles very slowly. The loss afterwards varied from 1/1 
to 4/1000 per kilometre. To reduce a pebble of 2 inche: 
diameter to 1 inch diameter—that is, to diminish its volume 
by seven-eighths—would require a journey of from about 219 to 
875 kilometres (approximately from 137 to 548 miles). Thi 
approximation, rough as it is, becomes still less exact as the 
pebbles decrease in size ; the rate of diminution in volume (ceter 
paribus) bearing a relation to the area of the surface. Thus thi 
smaller the pebble the further it will travel without materia 
diminution of size. Sand grains are even rounded with extreme 
X The waves of lakes also have some rounding effect, but this—excep' 
in the case of very large lakes, such as Lake Superior—is not important ; 
and such cases are, of course, not of common occurrence. | : { 3 
2 LT except floating pumice, cosmic dust, &c., as comparatively unimportan' 
