Fan, 19, 1871] 
Dr. Marshall Hall’s yacht Norna off the west coast of Europe, 
no animal forms have been discovered, so far as I am aware, 
identical with chalk fossils. Additional evidence has, however, 
been procured that over a large part of the bottom of the At- 
lantic a deposit is being formed mainly of disintegrated globi- 
gerinz and other foraminifera and coccoliths, which appears to 
be undistinguishable from the ancient chalk. 
Not fewer than twenty genera of vitreous sponges have been 
dredged belonging to two groups, the Hexactinellidze and the 
Lithistidee of Oscar Schmidt, both of which groups are highly 
characteristic of the chalk and greensand. These sponges are 
in vast numbers, like the ventriculites and their allies in older 
Cretaceous beds; and they, with other silicious organisms 
equally abundant in the modern chalk area, seem to be 
capable of supplying that amount of silica in a fine state 
of division which might explain the production of chalk 
flints. A large series of-echinoderms were found, recalling 
to a remarkable degree, from the profusion of Cidarids and 
of star-fishes of such genera as Archaster, Astrogonium, and 
Stellaster, the general facies of the chalk echinoderm fauna; 
and besides this general resemblence, members of several families 
have been recovered which were supposed to be extinct. Sa/eno- 
cidaris varispina A. Ag., dredged by De Pourtales in the Strait 
of Florida, is a living Salenia ; Zchinolampas caratomoides A. 
Ag. perpetuates some of the most marked characters of the 
Galeritidze. Pourtalesia, a genus first found by Count Pourtales 
and afterwards in the Porcupine expedition, is a true Dysaster. 
Porocidaris purpuratus, a fine species dredged off the Butt of 
the Lews, represents a genus hitherto known only by some 
isolated plates and radioles. Two very remarkable generic 
forms, dredged from the Porcupine off the coasts of Scotland 
and Portugal, only known from some fragments in the English 
white chalk, found a new family near the Diademide. Some 
new ophiurids approach their fossil ancestors ; and off the coast 
of Portugal the dredge brought up at one rich haul twenty or 
thirty examples of a fine Pentacrinus ; while over the whole area 
Rhizocrinus loffotensis, a degenerate little Bourgueticrinus, seem- 
ingly one of the last of the pear-encrinites, is abundant. 
I am not in a position to say much about those groups which 
T have not personally examined, except that Mr. Gwyn Jeffreys 
and Prof. Martin Duncan‘report that among the mollusca and 
corals many species occur which have been hitherto known only 
as fossils, principally, as might have been expected, in com- 
paratively sha!low water forms in the Tertiaries. 
I do not see that there is any object in attempting to explain 
this singular resemblance between these deep-sea deposits in the 
Atlantic and the old chalk in composition and structure and in 
embedded fauna on any other assumption than that of a conti- 
nuity of conditions over some part at all events of the area. 
During the lapse of time, while the fauna of shallower water has 
again and again undergone almost total change by changes in 
the distribution of temperature and in the distribution of sea and 
land, the fauna of the deep water has been also affected. Toa 
depth of 5,000 feet it is at present heated over a large portion of 
the North Atlantic many degrees above its normal temperature. 
Accepting, as I believe we are now bound to do, some form of 
the doctrine of the gradual alteration of species through natural 
causes, one is quite prepared to expect a total absence of the 
identical forms found in the old chalk. The utmost which might 
be anticipated is such a resemblance between the two faunz as 
might justify the opinion that the later fauna bears to the earlier 
the relation of descent with extreme modification. Sir Charles 
Lyell asks if we have dredged belemnites, ammonites, baculites, 
hamites, turrilites, &c. ; that question is, I think, best answered 
by the record of the old Cretaceous beds themselves, which are 
scarcely more remarkable for the presence of these singular and 
beautiful forms than for their rapid extinction. According to the 
view which I have felt myself compelled to adopt, the various 
groups of fossils characterising the Tertiary beds of Europe and 
North America represent the constantly altering fauna of the 
shallower portions of an ocean whose depths are still occupied 
by a deposit which has been accumulating continuously from the 
period of the pre-Tertiary chalk, and which perpetuates with 
much modification the pre-Tertiary chalk fauna. Ido not see 
how this view militates in the least against the ‘‘ reasoning and 
classification” of that geology which we have learned from Sir 
Charles Lyell. Our dredgings only show that these abysses of 
the ocean which Sir Charles Lyell admits in the passage quoted 
above to have outlasted on account of their depth a succession of 
geological epochs, are inhabited bya special deep-sea fauna 


NATURE 
SS ee ee ee 


227 
possibly as persistent in its general features as are the abysses 
themselves, WYVILLE THOMSON 

Ocean Currents 
ATTENTION has been much drawn of late to the subject of 
Ocean Currents and their causes, and it has occurred to me that 
there is a directing if not an originating cause of these streams, 
which has, so far as I am aware, been overlooked by physicists. 
It is known* that at some parts of the earth’s surface there exists 
an atmospheric pressure capable of sustaining a column of mercury 
in the barometer of upwards of 30 inches in height ; at the same 
time there are certain areas over which this pressure is only such 
as to raise the barometric column toa little over 29inches. Now 
if we compare the difference of absolute weight sustained by two 
such areas, we shall see that in the space over which the higher 
atmospheric pressure exists, there is an excess of weight of air, 
amounting in round numbers to 1,000,000 of tons on each square 
mile. Applying this fact to the region of the ocean in which 
the surface currents are best known, the North Atlantic, we find 
from the isobaric chart that there is throughout the year over a 
large portion of the eastern side of this sea, next the coast of 
North Africa, a pressure (to use the convenient mode of express- 
ing it) of upwards of 30°2 inches. To westward of this space, 
towards the Gulf of Mexico and the coast of the United States, 
the average pressure decreases; between Newfoundland and 
the British Isles the pressure is still diminished, till in the wide 
channel between Iceland, Norway, and Spitzbergen, we arrive 
at a yearly pressure of less than 29°6 inches. It is reasonable 
to believe that the waters which lie under the high pressure area 
have a tendency to escape from under the excessive weight, to- 
wards the space over which the pressure is less. But the high 
pressure area next the African coast is precisely that upon which 
the north-east trade winds descend, and the waters, aided in their 
choice of an exit, will naturally flow off to south-westward be- 
fore the wind. Their continuance in this direction is barred, 
however ; for across the whole of the southward passage between 
Africa and South America, there exists another belt of high 
pressure, out of which the south-easterly trade winds blow. 
The only course left for the escaping waters (allowing for the 
moment that the excess of pressure 7s a cause of their move- 
ment) is to westward, where the pressure is lessened, towards 
the Gulf of Mexico, and the east coast of America, and thence 
towards the low pressure space between Iceland and Norway. 
But this is exactly the course that the Gulf Stream, or rather the 
North Atlantic warm stream of which the Gulf Stream is the 
most prominent feature, is seen to take. Are we not then 
warranted in concluding that the difference of atmospheric 
pressure has some power both in originating and in directing 
the course of this ocean current ? 
In suggesting the unequal distribution of atmospheric pressure as 
a supplementary cause to difference of temperature and of density, 
to evaporation, rain, and winds, and to whatever further agents 
there may be in the production of ocean currents, I would 
venture to express a hope that some one in authority, by care- 
fully comparing and valuing the power of each one of these motive 
forces, and their application to the known streams, will give to 
the world a system of the causes of ocean currents which will be 
vastly more relevant to the phenomena these streams are known 
to present, than any one of the theories which have as yet been 
put forth. 
When we know that Sir John Herschel gives to the winds the 
entire right of setting the ocean streams in motion; that Captain 
Maury holds the universal circulation of the sea to be caused by 
nothing else than the differences in its specific gravity, and that 
Dr. Carpenter (or rather Professor Buff) would bring about a 
general interchange of polar and equatorial water by the aid of 
sunshine and frost alone; is it not time to ask which of these 
three causes we should accept as the true one, or if all three are 
partially concerned, what part is to be taken from each to let the 
others have their fair share in the work ? 
KEITH JOHNSTON, Jun, 
The Measurement of Mass 
I aM happy to meet with an opponent who comes so directly 
to the point as my Reviewer, W. M. W. 
* I would refer those who desire to look more particularly into this matter 
to the monthly isobaric charts prepared by Mr, Buchan, to illustrate his 
admirable paper on the mean pressure of the atmosphere, Trans. Royal 
Soc. Edin,, vol. xxv, 
