PHYSICS AND MATHEMATICS TO GEOLOGY. 151 
crust, when the tension caused by the excessive strain (and of which 
the first order of movement is an index) overcomes the resistance, and 
fractures and doubles up the strata;” and he adds: ‘‘ Mountain ranges 
are in fact the concluding term of the stress which caused the deforma- 
tion of the crust, and the movements which at those times took place 
must have been influenced by the greater energy of the strains then at 
work” (p. 546). It is difficult to see here what is intended to be cause 
and what effect. In fact, while a number of terms are employed which 
in mathematics and physics have a fairly definite meaning, I must con- 
fess my inability to form an adequate conception of what is meant by 
the passage as a whole. 
Prof. Prestwich refers (pp. 545, 544) to the hypothesis of the late 
Prof. KE. Roche (in the reference to which a misprint gives the year 1861 
for 1881) as supplying something of the kind of earth he wants. Thus 
an examination of Prof. Roche’s work* may be of some service. 
He supposes the earth to consist of a central nucleus or “ bloe,” 
homogeneous but for a possible accumulation of matter of greater 
density at the center, and of a superficial layer of lighter material. Of 
the nucleus, with the possible exception of a small core of heavier mat- 
ter, he says, ‘Sa densité caleulée, de 7 a 7:5, indique qwelle est mé- 
tallique, sans doute formée de fer - - -” The specific gravity of the 
heavier matter which may possibly exist at the center is, he says, ‘ cer- 
tainement bien inférieur {to 15], probablement 10 ou 12 (argent, plomb),” 
p. 235. The outer layer or crust he supposes to have a specific gravity 
about 3, and a thickness of about one-sixth the earth’s radius. Be- 
tween the crust and the nucleus there exists, it may be everywhere or 
only locally, molten matter such as appears at the surface in voleanie 
outbursts, but the total volume occupied by this must be small. Prof. 
Roche takes three results as given, viz, the earth’s total mass, the eecen- 
tricity of its surface, and the ratio of the principal moments of inertia, 
the last quantity being deduced from astronomical data. He satisfies 
all the conditions he recognizes by the aid of the following hypothesis 
regarding his nucleus: ‘“‘Ce bloe a pris sa forme définitive sous Vinflu- 
ence dune rotation moins rapide quelle West aujourd’hui, et il a con- 
servé laplatissement correspondant, malgré les accroissements sue- 
cessifs de vitesse du systeme résultants de sa contraction progressive ” 
(p. 232). In other words, he assumes the nucleus to have solidified 
before the crust and that it retains its shape unaltered. Thus as he 
regards the angular velocity as increasing in consequence of the dimi- 
nution in the moment of inertia through contraction in cooling, the nu- 
cleus possesses a smaller eccentricity than the erust. He supposes 
only a small difference in the length of the day at the dates of the two 
solidifications, so that the difference between the eccentricities of the 
nucleus and crust is also small. This however in no way justifies his 
“Académie - - - de Montpellier Mémoires de la Section des Sciences, 1880-1884, tome 
dixiéme, pp. 221-264. 
