226 

popular delusion as to the bearing of the new facts on geological 
reasoning and classification.” I do not say that the phrase ‘‘we 
are still living in the Cretaceous epoch ” is defensible in a strictly 
scientific sense, chiefly because the terms ‘‘ geological epoch ” 
and “ geological period” are thoroughly indefinite. We speak 
indifferently of the ‘‘Silurian period” and of the ‘Glacial 
period,” without consideration of their totally unequal 
value, and of the ‘‘ Tertiary period,” and of the ‘‘ Mio- 
cene period,” although the one includes the other. 
ntended rather, I believe, in a popular sense, to meet what 
seems to be the general popular impression, that a geological 
period has, in the region where it has been studied and defined, 
something in the shape of a beginning and an end, that it is 
bounded by periods of change, —elevation, denudation, or some 
other evidences of the lapse of unrecorded time; and that it would 
be inadmissible to speak of two portions of the same continuous 
deposit, however distant the times of their deposition might be, 
and however distinct their imbedded faunze, as belonging to two 
geological periods. 
Tt was certainly under this idea that in an address to a popular 
audience in April 1869,.I stated my belief that it is not only 
chalk which is being formed at present in the bed of the Atlantic, 
“but the chalk, the chalk of the Cretaceous period.” Sir Charles 
Lyell says, in summing up his objections at the end of the 
paragraph, ‘‘the reader will at once perceive that the present 
Atlantic, Pacific, and Indian Oceans are geographical terms, 
which must be wholly without meaning when applied to the 
Eocene, ard still more to the Cretaceous period; so that to talk of 
the chalk having been uninterruptedly forming in the Atlantic 
from the Cretaceous period to our own, is as inadmissible in a 
geographical as in a geological sense.” confess I do not under- 
stand the geographical difficulty ; the ‘* Atlantic Ocean ” is doubt- 
less a geographical term, but the depression under discussion 
occupies the area at present expressed by that term, and to use it 
seems to be the simplest way of indicating its position. That it 
is highly probable that the chalk has been so uninterruptedly 
forming over some parts of that area, is, however, exactly what I 
wish to show. I will therefore set aside the question of expression, 
and address myself simply to the consideration of the fact. And 
first with reference to the physical aspects of the case. 
All the principal axes of elevation in the north of Europe and 
in North America have a date long anterior to the deposition of 
the Tertiary and even of the newer Secondary beds ; and these 
strata were, consequently, all deposited with a certain relation in 
position to certain main features of contour, which are main- 
tained to the present day. Many oscillations have, undoubtedly, 
taken place since, and every spot on the European plateau has 
probably many times alternated between sea and land, but it is 
difficult to show that these oscillations have occurred in the 
latitude of Britain to a greater extent than 1,500 feet up and 
down ; a subsidence to that extent would, however, be sufficient 
to produce over most of the northern Jand a sea 100 fathoms 
deep, the average depth of the German Ocean. 
From a glance at a geological map of Europe and North 
America, it would seem that the sum of these elevations and 
subsidences has produced a gradual elevation of the edges, and 
a general contraction, of a basin the long axis of which coincides 
roughly with the axis of the Atlantic. The Jurassic beds crop 
out along the outer edge of this basin, the Cretaceous beds form 
a middle band, while the Tertiaries occupy the troughs and 
valleys. All of these, however, maintain a certain parallelism, 
- determined by the contour of the older mountain ridges, to one 
another and to the shores of the present sea. 
From the parallel of 55° N. lat., at all events to the equator, 
we have on either side of the Atlantic a depression 600 to 700 
miles in width, averaging 15,000 feet in depth. 
These two valleys are separated by the modern volcanic 
plateau of the Azores. I cannot think it at all probable that any 
general oscillations have taken place in the northern hemisphere 
since the commencement of the Tertiary period sufficient to form 
that immense abyss, or, if formed, to convert it into dry land ; 
but on this point Iam able to quote the highest authority :—“ If 
at any former period the climate of the globe was much warmer 
or colder than it is now, it would have a tendency to retain that 
higher or lower temperature for a succession of geological epochs. 
. . + The slowness of climatal change here alluded to, would 
arise from the great depth of the sea as compared to the height 
of the land, and the consequent lapse of time required to alter 
the position of continents and great oceanic basins... . The 
mean height of the land is only 1,000 feet, the depth of the sea 
NATURE 
It is | 


| Fan. 19, 1871 

15,000 feet. The effect, therefore, of vertical movements equally 
1,000 feet in both directions, upward and downward, is to cause a 
vast transposition of land and sea in those areas which are now con- 
tinental, and adjoining to which there is much sea not exceeding 
1,000 feet in depth. But movements of equal amount would have 
no tendency to produce a sensible alteration in the Atlantic or 
Pacific Oceans, or to cause the oceanic and continental areas 
to change places. Depressions of 1,000 feet would submerge 
large areas of the existing land, but fifteen times as much move- 
ment would be required to convert such land into an ocean of 
average depth, or to cause an ocean three miles deep to replace 
any one of the existing continents.” (Lyell, “Principles of 
Geology,” 1867, pp. 265-6.) The wide extent of Tertiaries in 
Europe and the north of Africa sufficiently proves that much 
land has been gainedin Tertiary and post-Tertiary times, and the 
great mountain masses of Southern Europe give evidence of great 
local disturbance. But although the Alps and the Pyrenees are 
of sufficient magnitude to make a deep impression upon th 
senses of men, taking them together, their materials would, it 
spread out, only cover the surface of the North Atlantic to the 
depth of about six feet, and it would take at least 2,500 times as 
much to fill up its bed. It would seem by no means improbable 
that while the edges of what we may call the great Atlantic de- 
pression have been gradually raised, the central portions may 
have acquired an equivalent slight increase in depth ; but it is 
most unlikely that while the main features of the contour of the 
northern hemisphere remained the same, an area of so vast extent 
should have been depressed by more than the height of Mont 
Blanc, On these physical grounds alone I should be inclined to 
believe that a considerable portion of this area has been con- 
tinuously under water, and that consequently a deposit has been 
forming uninterruptedly from the period of the chalk to our own. 
I would now refer to the palzeontological bearings of the ques- 
tion. Sir Charles Lyell says (p. 263), ‘‘ The reader should be 
reminded that in geology we have been in the habit of founding 
our great chronological divisions, not on foraminifera and sponges, 
nor even on echinoderms and corals, but on the remains of the 
most highly organised beings available to us, such as mollusca, 
. . . Indealing with the mollusca, it is those of the highest or 
most specialised organisation which afford us the best characters 
in proportion as their vertical range is the most limited. Thus 
the cephalopoda are the most valuable, as having a more re- 
stricted range in time than the gasteropoda, and these again are 
more characteristic of the particular stratigraphical sub-divisions 
than are the lamellibranchiate bivalves, while these last again are 
more serviceable in classification than the brachiopoda, a still 
lower class of shell-fish, which are the most enduring of all.” 
With great deference to Sir Charles Lyell, I cannot regard the 
most highly specialised animal groups as those most fitted to 
gauge the limits of great chronological divisions, though I admit 
their infinite value in determining the minor sub-divisions. 
The culmination of such animal groups, such as we find in the 
marvellous abundance and variety of both orders of cephalopods 
at the end of the Jurassic and the commencement of the Cre- 
taceous period, undoubtedly brings into high relief, and admir- 
ably illustrates to the student, the broad distinctive characters of 
the Mesozoic fauna ; but speaking very generally, the more highly 
a mollusc is specialised, the shallower is the water which it in- 
habits. The cephalopods are chiefly pelagic and surface 
things, and their remains are consequently found in deposits from 
all depths. The gasteropods, with comparatively few exceptions, 
range from the shore to 1 to 200 fathoms, and lamellibranchs 
become scarce at a slightly greater depth ; while some orders of 
crustacea, brachiopods, echinoderms, sponges and foraminifera, 
descend in scarcely diminished numbers to a depth of 10,000 
feet. In fact, the bathymetrical range of the various groups in 
modern seas corresponds remarkably with their vertical range 
in ancient strata. 
A change in the distribution of sea and land involving a mere 
change in the course of an ocean current, might modify the con- 
ditions of an area for all cephalopods, pteropods, heteropods, 
and other surface living animals of high type, even to their ex- 
tinction. By oscillations of 500 feet up and down, the great mass 
of gasteropods and all reef-building corals, would be forced to emi- 
grate, become modified, or destroyed, andanother hundred fathoms 
would exterminate the greater number of bivalves ; while eleva- 
tions and depressions to ten times that amount might only slightly 
affect the region of brachiopods, echinoderms, and sponges. 
In the late deep-sea dredgings by M. de Pourtales, off the 
American coast, and by H.M.S, Lightning and Porcupine, and 
