28 
NATURE 
| May 11, 1871 

the other one is ‘‘ Fe/is — 14. Femur Right side.” There is no 
record in the catalogue by whom they were presented, nor of 
any of the circumstances of their g/sement. The specimens, in 
fact, have no history whatever, and I can only say that I found 
them in close juxtaposition with a large series of red-deer bones 
from Holderness, with which they perfectly agree in their minera- 
logical condition. I have no doubt that they are dond fide from 
the Holderness Peat. 
Their identification as bones ot F, o (variety spelaa) is also 
certain. 
Hull Royal Institution C. CARTER BLAKE 
Eozoon Canadense 
SINCE reading some of the communications on the Eozo6n, 
which have appeared from time to time in NATURE, I have felt 
constrained briefly to give the results of my examination of the 
“Fozoic ” limestone in Eastern Massachusetts. I am the more 
disposed to do this, hoping that a new line of investigation will 
be suggested to observers in other localities. 
Last autumn I visited for the first time the quarries of 
“*Fozoon” limestone in Chelmsford, under the guidance of my 
friend Mr. Burbank, of Lowell, Massachusetts, who has fur- 
nished many microscopists with specimens for sections. Having 
been long engaged in the study of the foliated series of rocks, 
and having years ago discovered indubitable evidence that por- 
uons of the included limestone are cf vaporous origin, I was 
prepared to recognise the same feature in the Chelmsford 
“* Fozoic ” rock. I was accordingly not surprised on examination 
to find, what the advocates of the organic nature of the Kozoén 
seem never to have suspected, that the limestone in question is 
not a ‘sedimentary rock ;” that it occupies, or rather occupied, 
(for it has been for the most part removed) pockets or oven- 
shaped cavities, which were once plainly overarched by gneiss ; 
that it is foliated, there being a regular succession of leaf-like 
layers from the walls toward the centres of the cavities, witness 
to which is borne by a like succession of different minerals ; that 
in some places it ramifies the surrounding rock in a vein-like 
way, while in others it exactly conforms with the most abrupt 
irregularities of surface ; that in one locality, which I have re- 
peatedly examined, it conforms with the uneven portions of a 
mass of syenite, with which it is so associated as to reveal its 
more recent origin ; and that, therefore, it is not of nummulitic 
derivation, but was deposited in a vein-like form, the materials 
having been probably forced up into the cavities from below 
while in a vaporous state. 
Such, in few words, is the result of my examination—a result 
which tends to show that the ‘‘Eozodn”’ of Eastern Massa- 
chusets is not organic, and that thus it belongs to the department 
of Mineralogy, and not to that of Paleontology. Waving ad- 
ditional particulars for the present, I may simply add that I 
propose in due time to give a detailed exposition of the relations 
of this famous ‘‘ Eozoic” rock, 
Cambridge, Mass., April 15 Joun B. PERRY 


THICKNESS OF THE EARTH’S CRUST 
] SEE that at p. 296 of your journal for February last, 
which has recently reached Calcutta, you print a 
lecture by Mr. David Forbes “On the Nature of the Earth’s 
Interior,” in which reference is made to the Mr. W. Hop- 
kins’s method of determining whether the thickness of 
the earth’s crust is great or small when compared with 
the whole radius, and to M. Delaunay’s objection to it. 
The lecturer refers to me as having approved of Mr. 
Hopkins’s method, which I always have done and do 
still, and then makes the following apparently crushing 
remarks to annihilate Mr. Hopkins and all who approve 
of his method and of the result to which it leads, 
viz., that the crust is very thick. He says :—“M, 
Delaunay, an authority equally eminent as a mathe- 
matician and an astronomer, was induced to undertake 
the reconsideration of the problem; a labour (!) which 
has resulted in altogether reversing the above decision 
and demonstrating the complete fallacy of the premises 
upon which so much elaborate reasoning had been ex- 
pended,” 

As the lecturer had condescended to mention my name 
in connection with the subject, I wonder why he has 
taken no notice of my letter in reply to M. Delaunay, 
which was printed in your journal for July 1870, six months 
before the lecture was delivered, and which also appeared 
about the same time in the P/Az/osophical Magazine and the 
Geological Magazine. In this I showed that M. Delaunay 
had evidently misconceived the problem, and that Mr. 
Hopkins’s method is altogether unaffected by his remarks. 
So much has been said about profound mathematical 
calculations in connection with Mr. Hopkins’s investi- 
gation, that I conceive many have shrunk from attempting 
to understand the question at issue, from a feeling that they 
would not be able to comprehend it were they to attempt 
todoso. But this is quite a mistake. Anyone with an 
ordinary degree of knowledge of popular astronomy and 
of mechanical action is quite competent to form a good 
opinion on the point in dispute. What Mr. Hopkins did 
may be divided into two parts. He first conceived an 
idea, which was to be the basis of his calculation ; and 
then he made his calculation. Itis the calculation that 
calls for the “profound mathematics.” But it is not this 
that is the matter of dispute. Itis the z¢ea, on which the 
calculation is based, which M. Delaunay calls in question. 
I think I can make the matter sufficiently plain to your 
readers to enable them to form their own opinion. 
Everyone having a knowledge of popular astronomy is 
aware that the earth revolves round an axis, which is fixed 
in the earth’s solid crust, but shifts very slowly in space, 
producing what has been known ever since the days of 
Hipparchus by the name Precession. On this fact as his 
ground-work Mr. Hopkins reasoned as follows ; and so got 
to his zdea, which formed the basis of his calculation. 
Suppose the earth has a solid crust, the interior being filled 
up with fluid. If the axis remained steady in space and 
the crust revolved round it uniformly, no doubt, although 
the crust and fluid may have moved differently at one 
time, yet in the lapse of ages friction and viscosity in the 
fluid would cause the fluid at last to revolve with the crust 
just as if the whole were one solid mass. This being the 
case, suppose a slight horizontal push is given to the two 
poles, in opposite directions, so as slightly to shift the 
axis in space ; what would happen? The revolving crust, 
by this new and additional motion, would slip over the 
surface of the revolving fluid, through a small space pro- 
portionate tothe push given to the poles. The fluid could 
not possibly acquire in an instant this new motion, how- 
ever small it might be, because the fluid is not rigidly 
connected with the crust. Suppose a second, and a third, 
and a succession of slight horizontal pushes to be given to 
the poles in a continually altering direction, the effect will 
be that the revolving crust will be continually slipping 
over the revolving fluid which has not time to acquire 
these new motions given instantaneously to the solid crust. 
These successive slight pushes given to the poles, and so 
to the solid crust, represent the unceasing action upon the 
crust of the force which causes the motion of precession 
in the earth’s axis, and arises from the attraction of the 
sun and moon on the protuberant parts of the earth about 
the equator. 
Mr. Hopkins having reasoned thus far, went a step 
farther, and so came to his fundamental idea. He saw 
that the thinner the crust the smaller would be tht mass 
which the disturbing force producing precession woud have 
tomove, and therefore the greater would be the notion 
caused, that is, the precession. Here, then, he disterned 
a connecting link between the amount of precesson of 
the earth’s axis and the thickness of the earth’s rust, 
This was the zdea I have aliuded to. 
Starting from this idea he entered upon a prdound 
calculation and obtained a formula, which gives thethick- 
ness in terms of the amount of precession. This anount 
is a matter of observation ; and the thickness canthere- 
fore be deduced by the formula from the observet pre- 
