414 
existing causes which forms the ba-is of uniformitarianism in 
geology, and which had to go on with various and great modi- 
fications of detail, through the successive stages of the geological 
history, till the land and water of the northern henisphere 
attained to their present complex structure. . 
So soon as we have a circumpolar belt or patches of Eozoic 
(or Archzean, or pre-Cambrian, if these terms are preferred) 
land, and ridges running southward from it, we enter on new and 
m re complicated methods of growth of the continents and seas. 
Here we are indebted to Le Conte for clearly pointing out that 
our original Eozoic tracts of continent were in the earliest times 
areas of deposition, and that the first elevations of land out of 
the primeeval ocean must have differed in important points from 
all that have succeeded them ; but they were equally amenable 
to the ordinary laws of denudation. Portions of these olcest 
crystalline rocks, raised out of the protecting water, were now 
eroded by atmospheric agents, and especially by the carbonic 
acid, then existing in the atmosphere perhaps more abundantly 
than at present, under whose influence the hardest of the gneissic 
rocks gradually decay. The Arctic lands were subjected in 
addition to the powerful mechanical force of frost and thaw. 
Thus every shower of rain and every swollen stream would carry 
i to the sea the products of the waste of land, sorting them into 
fine clays and coarser sands ; and the cold currents which cling 
to the ocean bottom, now determined in their courses, not merely 
by the earth’s rotation, but also by the lines of folding on both 
sides of the Atlantic, would carry south-westward, and pile up 
in marginal banks of great thickness, the debris produced from 
the rapid waste of the land already existing in the Arctic regions. 
Yhe Atlantic, opening widely to the north, and having large 
rivers pouring into it, was especially the ocean characterised, as 
time advanced, by the prevalence of these phenomena. Thus 
throughout the geological history it has happened that, while the 
middle of the Atlantic has received merely organic deposits of 
shells of Foraminifera and similar organisms, and this probably 
oaly toa small amount, its margins have had piled upon them 
beds of detritus of. immense thickness. Prof. Hall, of Albany, 
wis the first geologist who pointed out the vast cosmic import- 
ance of these deposits, and that the mountains of both sides of 
the Atlantic owe their origin to these great lines of deposition, 
a'ong with the fact, afterwards more fully insisted on by Rogers, 
that the portions of the crust which received these masses of 
debris became thereby weighted down and softened, and were 
more liable than other parts to lateral crushing.! 
Thus in the later Eozoic and early Paleozoic times, which 
succeeded the first foldings of the oldest Laurentian, great ridges 
were thrown up, along the edges of which were beds of lime- 
stone, and on their summits and sides thick masses of ejected 
igneous rocks. In the bed of the central Atlantic there are no 
such accumulations. It must have been a flat, or slightly ridged, 
plate of the ancient gneiss, hard and resisting, though perhaps 
with a few cracks, through which igneous matter welled up, as 
in Iceland and the Azores in more modern times. In this con- 
dition of things we have causes tending to perpetuate and extend 
the distinctions of ocean and continent, mountain and plain, 
already begun ; and of these we may more especially note the 
continued subsidence of the areas of ‘greatest marine deposition. 
This has long attracted attention, and affords very convincing 
evidence of the connestion of sedimentary deposit as a cause 
with the subsidence of the crust.” 
We are indebted to a French physicist, M. Faye (Reve S-ten- 
‘/figue, 1836), for an important sugzestion on this subject. It is 
that the sediment accumulated along the shores of the ocean pre- 
u The connection of accumulation with subsidence was always a familiar 
consideration with geologists ; but Hall seems to have been the first to state 
its true significance as a geological factor, and to see that those portions of 
the crust which are weighted down by great detrital accumulations are neces- 
sarily those which, in succeeding movements, were elevated into mountains. 
Other American geologists, as Dana, Rogers, Hunt, Le Conte, Crosby, &c., 
have followed up Hall’s primary suggestion, and in England, Hicks, Fisher, 
Starkie Gardner, Hull, and others, have brought it under notice, and it 
enters into the great generalisations of Lyell on these subjects. 
_ Dutton in * Report of U.S. Geological Survey,” 1881. From factsstated 
in this report and in my ‘‘ Acadian Ge slogy,’’ it is apparent that in the 
Western States and in the coal-field of Nova Scotia shallow-water deposits 
have been laid dowr up to thicknesses of 10,000 to 20,000 feet in connection 
with continuous subsidence. See also a paper by Ricketts in the Geod. Wag. 
1883. It may be well to add here that this doctrine of the subsidence of wide 
areas being caused by deposition does not justify the conclusion of certain 
glacialists that snow and ice have exercised a like power in glacial periods. 
In truth, as will appear in the sequel, great accumulations of snow and ice 
require to be preceded by subsidence, and wide continental areas can never 
be covered with deep snow, while of course ice can cause no addition of 
weight to submerged areas. 
NATURE 
[Sept. 2, 1886 
sented an obstacle to radiation, and consequently to cooling of 
the crust, while the ocean floor, unprotected and unweighted, 
and constantly bathed with currents of cold water, having great 
power of convection of heat, would be more rapidly cooled, and 
so would become thicker and stronger. This suggestion is 
complementary to the theory of Prof. Hall, that the areas 
of greatest deposit on the margins of the ocean are necessarily — 
those of greatest folding and consequent elevation. We have 
thus a hard thick resisting ocean bottom which, as it settles 
down toward the interior, under the influence of gravity, squeezes 
upward and folds and plicates all the soft sediments deposited on 
its edges. The Atlantic area is almost an unbroken cake of this 
kind. The Pacific area has cracked in many places, allowing 
the interior fluid matter to ooze out in volcanic ejections. 
It may be said that all this supposes a permanent continuance 
of the ocean basins, whereas many geologists postulate a mid-— 
Atlantic continent! to give the thick masses of detritus found in 
the older formations both in Eastern America and Western 
Europe, and which thin off in proceeding into the interior of 
both continents. I prefer with Hall to consider these belts of 
sediment as in the main the deposits of northern currents, and 
derived from Arctic land, and that like the great banks of the 
American coast at the present day, which are being built up by 
the present Arctic current, they had little to do with any direct 
drainage from the adjacent shore. We need not deny, however, 
that such ridges of land as existed along the Atlantic margins 
were contributing their quota of river-borne material, just as on 
a still greater scale the Amazon and Mississippi are doing now, 
and this especially on the sides toward the present continental 
plateaus, though the greater part must have been derived from 
the wide tracts of Laurentian land within the Arctic Circle or | 
near to it. It is further obvious that the ordinary reasoning 
respecting the necessity of continental areas in the present ocean 
basins would actually oblige us to suppose that the whole of the 
oceans and continents had repeatedly changed places. This 
consideration opposes enormous physical difficulties to any 
theory of alternations of the oceanic and continental areas, 
except locally at their margins. I would, however, refer you 
for a more full discussion of these points to the address to be 
delivered to-morrow by the President of the Geological Section. 
But the permanence of the Atlantic depression does not ex- 
clude the idea of successive submergences of the continental 
plateaus and marginal slopes, alternating with periods of eleva- 
tion, when the ocean retreated from the continents and con- 
tracted its limits. In this respect the Atlantic of to-day is much 
smaller than it was in those times when it spread widely over 
the continental plains and slopes, and much larger than it has 
been in times of continental elevation. This leads us to the 
further consideration that, while the ocean beds have been sink- 
ing, other areas have been better supported, and constitute the 
continental plateaus ; and that it has been at or near the junc- 
tions of these sinking and rising ares that the thickest deposits 
of detritus, the most extensive foldings, and the greatest ejec- 
tions of volcanic matter have occurred. There has thus been a 
permanence of the position of the continents and oceans through- 
out geological time, but with many oscillations of these areas, 
producing submergences and emergences of the land. In this 
way we can reconcile the vast vicissitudes of the continental 
areas in different geological periods with that continuity of de- 
velopment from north to south, and from the interiors to the 
margins, which is so marked a feature. We have for this reason 
to formulate another apparent geological paradox, namely, that 
while in one sense the continental and oceanic areas are perma- 
nent, in another they have been in continual movement. Nor 
does this view exclude extension of the continental borders or of 
chains of islands beyond their present limits, at certain periods ; 
and indeed the general principle already stated, that subsidence 
of the ocean bed has produced elevation of the land, implies in 
earlier periods a shallower ocean and many possibilities as to. 
volcanic islands, and low continental margins creeping out into 
* Among American geologists, Dana and Le Conte, though from some- 
what different premises. maintain continental permanence. Crosby has 
argued on the other side. In Britain, Hull has elaborated the idea of inter- 
change of oceanic and continental areas in his memoir in Tyas, Dublin 
Society, and in his work entitled ‘‘The Physical History of the British 
Islands.” Godwin-Austen argues powerfully for the permanence of the At- 
lantic basin, Q. /. Geol. Soc. vol. xii. p. 42. Mellard Reade ably advocat 
the theory of mutation. The two v.ews require, in my judgment, to b 
combined. More especially it is necessary to take into the account the exist- 
ence of an Atlantic ridge of Laurentian rock on the west side of Europe, o} 
which the Hebrides and the oldest rocks of Wales, Ireland, Western France, 
and Portugal are remnants. 
