48 Correspondence—Rev. M. H. Close. 
published at intervals running over sixteen years, on the origin of, 
and on changes in, serpentine, chrysotile, and other related minerals. 
Nor is there the least notice of any of the phenomena which we 
ascribe to chemical changes (methylosis) in rocks. 
My Memoir on Jointing and Slaty Cleavage, published in the 
Transactions of the Royal Irish Academy, vol. xxv. pp. 605-662, 
1875, is altogether unnoticed. 
I have in the last place to mention that all reference to my 
Monograph of the Permian Fossils of England, also some subjects 
introduced, are omitted in the Text-Book; although the latter 
contains (p. 752) some figures of shells copied from it, but without 
acknowledgment. 
Under the conviction that Dr. Geikie doubtless desires his Teat- 
Book to contain fair and correct references to the labours of his 
colleagues on the subjects he has touched upon, I am disposed to 
believe that he will not overlook these notes when preparing, as 
pretty certainly will be the case, another edition of his valuable work. 
Witiram Kine. 
GLENOIR, NEAR GaLway, Nov. 5, 1882. 
THE RIGIDITY OF THE EARTH. 
S1r,—In his letter in the November No., 1882, «‘On the Depression 
of Ice-loaded Lands,” the Rev. O. Fisher laments the disagreement 
between the mathematical physicists and the geologists respecting 
the rigidity of the body of the earth. But if the earth have a viscous 
rigidity, there need be no incompatibility between their respective 
contentions. The character of a viscous solid is that, though it may 
seem to be quite rigid when tested by a short-lasting stress, it may be 
capable of yielding very considerably, in some cases almost indefi- 
nitely, to a much smaller stress continued for a sufficiently long 
time. Sir William Thomson’s conclusion as to the steel-rigidity 
of the earth is founded upon the magnitude of the short-period ocean 
tides, classing with these even the monthly tides due to the ellipti- 
city of the moon’s orbit (he has given up the argument from pre- 
cession). But he himself declared in his Address to the Physical 
Section of the British Association, at Glasgow, in 1876, that the- 
absence of any indication of a 18-6-year ocean tide, depending on 
the revolution of the moon’s nodes, could not be easily explained 
without assuming or admitting a considerable degree of yielding in 
the body of the earth. That is to say, our earth, as a whole, is a 
viscous, or practically viscous, solid, which, notwithstanding its 
apparently very high rigidity when tried by reciprocating stresses of 
short period, may be able to yield to the full satisfaction of geologists 
to a sufficiently long-continued pressure, such as that of a great 
depth of ice in high latitudes during the Glacial period. I omit 
some other considerations which go to strengthen this conclusion. 
M. H. Cross. 
University Crus, Dustin, Nov. 7, 1882. 
