286 
NATURE 
[Fed. 9, 1871 

LETTERS TO THE EDITOR 
[The Eaitcr does not holt himself responsible for opinions expressed 
by his Correspondents. No notice is taken of anonymous 
communications, | 
The Cretaceous Period 
In the number of NATureE for January 19 appeared a letter 
from Prof Wyville Thomson, in which he attempts to justify the 
opinion which has been concisely summed up in the words ** we 
are still living in the Cretaceous epoch,” a statement emphatically 
contested in the lately published ‘‘ Students’ Elements of Geo- 
logy,” by Sir Charles Lyell. 
Prof. Thomson begins by supposing that the term Cretaceous 
epoch is considered by geologists as an undefined period of time 
which may very well be elastic enough to 1each up to the present 
day ; and, in this misapprehension of the exact meaning of 
geological terms, betrays the source of the whole misunderstani- 
ing. This accounts for an unnecessary digression about axes 
of elevation and raising the floor of the Atlantic to the surface ; 
as if the possibility of part of that ocean having remained as such 
since the chalk period had ever been denied. It is known with 
certainty that a large area of Cretaceous sea existed where the 
Atlantic dues not extend, but beyond our observations on dry 
land it is perfectly useless to speculate how far the present distri- 
bution of land and water coincides with that which obtained 
when Belemnitella mucronata was a livinganimal. Moreover the 
possibility of perfectly continuous and conformable series of 
deposits from the chalk period upwards being present somewhere 
at the bottom of the sea, has not received an atom of confirma- 
tion in the late deep-sea dredgings, and if true, would be no 
reason fur laying aside a very serviceable classification based on 
other than purely stratigraphical considerations. 
In speaking of organic remains, Prof. Thomson persists in 
misunderstanding the reasons on which the title ‘*Creta- 
ceous”” has been founded, and sv clearly defined in the contro- 
verted paragraph of the Student’s Elements, p. 263. He cannot 
gainsay the fact thatse many genera of Cephalopvda have com- 
pletely disappeared, and, indeed, grants only a generic and specific 
resemblance between ceriain chalk fossils anc recent deep-sea 
forms ; which is obviously no adequate reason for the assumption 
that we now live in the Cretaceous epoch. It seems also to have 
been forgotten that our knowledge of this period is not confined 
to chalk strata, but is derived also from the examination of sand 
and clay formations in England, as well as those on the Continent 
and America, which are recognised as Cretaceous from their con- 
taining the same species of moilusca which characterise our 
chalk. Further, as the only organisms comnon to the Atlantic 
sand and the chalk are the Globigerina and other Bathybius, 
which are also common to the Indian and Pacific Oceans, any 
inferences drawn from their occurrence in the Atlantic are equaily 
applicable to the two latter cases, and equally so to Eocene seas 
where Globigerinze flourished along with Nummulites. 
In fact the whole question is shorily this: calcareous mud has 
been dredged up from the bottom cf the Auantic, often from creat 
depths. This mud resembles cl alk so much as to leave no doub: but 
that the older deposit was produced under identicaliy the sime 
physical circumstances and organic conditions (as far as concern 
the matrix of Globigeiinze, Coccoliths, &c., but exclusive of all 
imbedded remain:) as the newer formation. ‘The outward re- 
semblance of the two rocks, and the fact of the new deposit 
hiving been found so near where the old one accumulated, 
together with the recurrence of a form of Encrinite anda few 
Echinoderms, have imposed on Dr. Thomson, and led him to a 
conclusion which has no justification in tacts. Had this eminent 
naturaiist been an equally experienced geologist, he would have 
seen that, with the recurrence of outward conditions, it was but 
natural to expect a return of some old forms of life, as is the real 
state of the case, Tir tes 
A QUARTER of acentury ago, when first I began to study 
geology, it appeared to me that a predomimance was given to 
the more recent rocks, such as the Picistocene, Miocene, Lucene, 
Cretaceous, and the like, to which they were not entitled, when 
ranked as periods alongside such gieat groupsas the Carboni erous 
the Silurian, the Cambro-silurian, and the Cambrian, The more 
JT have since daily learned confirmed me in this opinion, therefore 
it is with great interest that I have watched the discussion on the 

valuable suggestions of Drs. Thomson and Carpenter. Perhaps 
I may be allowed to join in the controversy, and to draw atten- 
tion to some facts connected with the Carboniferous period, which 
seem to add weight tothe arguments put forward by those gentle- 
men. Jefore proceeding further, I ought to state that m the 
Carboniferous period Linclude the so-called Old Red sandstone or 
Devonian period, on account of their being interpenetrated, doves 
tailed, and graduated one into the other in the S. W, of Ireland, 
Inthe counties Limerick and Clare, well marked divisions 
occur in the Carboniferous period, of which the following is an 
epitome :— 
11. Coal-bearing rocks. 
Io. Flag series. 
9. Lower coal-measure shales. 
8. Upper limestone. 
Calp (shaly orgillaceous limestone, with shales and 
sometimes sandstones. ~ Between the calp and the 
upper limestone a cherty zone may occur, but it is 
not constant). 
. Upper chert zone. 
. Amorphous limestone (Fenestrella beds). 
Lower chert zone. 
Lower shaly limestone. 
. Lower limestone shale. 
1. Grit, &c. (Old Red sindstone, or Devonian period). 
Certain fossils do range through all these divisions, while others 
will occur in all the similarly, or nearly similarly, cireumstanced 
rocks ; nevertheless, the major part of, if not all, these divisions 
have their groups of fossils. In some, a fossil first appears, in 
others it dies out, so that any one well acquainted with the rocks 
in the field can tell from the assembly of fossils to what division 
of the Carboniferous period the rock belongs. 
The section of Limerick and Clare is only local, and will not 
be found in many places in Ireland. To the $.W. and S. in 
Cork and Kerry, some otf the divisions die out, while others 
thicken, so that eventually in S. W. Cork and Kerry the limestones 
have entirely disappeared. To the N., N.E., and E. of Clare 
other changes take place. Moreover, they are not as gradual or 
regu aras those to the S. W,, and more especially if the rocks are 
followed towards the N.E. In north Galway the divisions in 
the limestone have disappeared, while interstratified with the 
limestone, in some place evidently high up (the representation of 
division No. 8) will be found sandstone, and conglomerates in 
aspect exactly similar to the so-called Old Red sandstone, and 
when fossiliferous containing similar fossils. Farther N. 2, in 
the county Leitrim, the divisions are very similar to those in 
Clare, while still farther N.E. the different divisions become 
mixed, and in the coal-helds of Ulster will be found rocks similar 
to the coal measures, the Old Red sandstone, and the Carboniferous 
limestone, ail interstratified one with another. 
Neither are these rocks without breaks. In West Cork there is 
a continuous sequence either from the upper Silurian rocks, or 
rocks immediately above them to the coal measures ; while in 
Kerry, there are in these rocks tw» well-marked breaks or un- 
conformabilities, and in Limerick there may be another, as, at the 
only junciion exposed, between the Jower-coal-measure-shales 
and the upper-limestone, the limestone appears to have been 
denuded prior to the shales having been deposited. By the 
above it is shown that the condition under which the Carboniferous 
rocks were deposited, in the area that is now Ireland, are very 
varied in character, and if Great Britain were also included, 
other marked changes could be pciated out ; as, for instance, in 
South Staffurdshire, where there seem to have heen subaérial 
accumulations going on during a great part of the Carboniferous 
period, as suggested by the vast thickness of the coal. If, in 
such a mere speck us Ireland, these great changes took place in 
the Carboniferous period, what must have been the vast changes 
on the whole earth’s surface ? 
To me there appears to be a certain analogy between “‘the 
Carboniferous period” and the ‘‘ Cretaceous period of Thomson 
and Carpenter.” Both are great limestone periods, and both 
will have their coal beds. But what is more remarkable, Car- 
penter sugyests that the geologist of future ages will separate the 
rocks forming in the ** cold area” fiom the rocks forming in the 
“warm area,” and put them in separate formations on account 
of the great difference in their mineral constituents and their 
assembly of fossiis, while the greater number of the field geo- 
lozists of the present age make two periods of the rocks of the 
Carboniferous period (viz., Carboniferous period and Devonian 
period), although the rocks of both groups are found to inter- 
