94  Correspondence—Mr. 8. H. Wright—Rev. O. Fisher. 
advanced I cannot say; but all, I maintain, would be found wanting 
by a careful and competent judge. If any of your readers can throw 
light on the subject, it would be interesting to those geologists who 
happen to have observed the peculiarity to which I refer. 
CLevELAND Lopez, LowEr SyDENHAM. S. H. Wricut. 
The nature and origin of these Terraces is, we think, now gene- 
rally very well understood by geologists. 
We recommend to Mr. 8. H. Wright’s consideration an excellent 
little article upon them which appeared in the GroLocicaL MaGazINE 
for 1866, Vol. III. p. 293, by the late G. Poulett Scrope, Esq., F.R.S., 
F.G.S., than whom we could hardly cite a more competent observer 
or more trustworthy geological guide.—Hpir. Grou. Mae. 
THE) “RIGID EI) OE EGE, sR AucHaEe 
Srr,—It has given me much pleasure to read Mr. Close’s remarks 
referring to my lament over the disagreement between mathematical 
physicists and geologists touching the condition of the interior of 
the earth. His letter gives promise that a further discussion of the 
question with him may serve to elucidate it. 
Mr. Close has not, I think, exactly apprehended my meaning. I 
wrote of the conclusion arrived at by mathematicians, “that the 
earth is excessively rigid from its centre to its surface.” Mr. Close, 
on the other hand, writes of the disagreement between them and 
geologists respecting “the rigidity of the body of the earth.” It is 
important to be precise as to what we are discussing. As a physical 
geologist I seek to explain the phenomena exhibited by the masses 
which constitute the surface; its continents, mountains, plains, 
valleys, oceans, and volcanos. Still, these phenomena require us to 
speculate upon the condition of the interior down to a considerable 
depth; yet not necessarily to a depth which bears any large pro- 
portion to the entire radius. In short, I am willing to relegate the 
“body of the earth” to the physicist pure and simple, as a region 
beyond my province, and respectfully to accept his conclusion that 
it is extremely rigid. Possibly this rigidity may be no more than 
that viscous rigidity which Mr. Close so accurately describes. showing 
in his letter how such a condition of the interior would be capable 
of explaining many of the facts relied upon to establish rigidity. It 
certainly also appears to suit, better than absolute rigidity, with one 
to which he has not alluded; namely, that the present ellipticity of 
the earth agrees so well with the present period of diurnal rotation. 
I will now state some objections, which, on geological grounds, 
I would offer against the contention of Mr. Close, that a general 
viscous rigidity, such as I understand him to advocate, would meet 
the requirements of the problem; and I will point out one instance 
of the neglect of geological phenomena by a mathematician. I, 
maintain that the surface phenomena require that the cooled crust 
of the earth should be far more rigid than what it rests upon. For 
instance, they require that the substratum should be sufficiently fluid 
to admit of the crust being shifted over it towards the mountain 
ranges; that it should likewise be in a condition to flow upwards 
into narrow chasms, and form igneous dykes, and to furnish the 
