R. M. Deeley—LIsostasy. 3238 
to leave islands of enamel is one of the chief characters of 
Trogontherium in which it differs from Castor, and this feature is 
strikingly shown in the Copford tooth. Although only a com- 
paratively young tooth, as proved by the narrowness of the grinding 
surface compared with the remainder of the crown (Figure, 6), the 
single inner fold (a) extends only 5-5 mm. from the worn surface ; 
while of the three outer folds (c) the posterior one is already an 
Trogonthervum from Copford. Upper premolar 4 from left side. Nat. size. 
a, inner side, showing one fold of enamel; 6, front surface; c, outer side: 
arrows point to folds; d, grinding surface, showing enamel folds and 
posterior enamel island. The enamel at back of tooth has been broken 
away, and is here restored by dotted line. 
island (d@) disconnected from the outer wall of enamel, and the other 
two are nearly in the same condition, the anterior and largest one 
being only 1 mm. in depth. 
mm. 
Greatest length of tooth . : : é 28-7 
Greatest width of tooth . : ; 12-7 
Back to front of tooth . ‘ i ¢ 9-0 
Back to front of worn surface . f ; 8-5 
Width of worn surface . é i 5 10-0 
I have to thank my friend and colleague, Mr. Clement Reid, for the 
suggestion that I should examine the Copford ‘ Beaver’ and possibly 
confirm Richard Lydekker’s shrewd perception of its affinity to 
Trogonthervum. The results of my examination of the specimen are 
expressed in this note. 
VIII.—Isosrasy. 
By R. M. DEELEY, M.Inst.C.H., F.G.S. 
URING recent years the question of the conditions of stress and 
strain in the earth’s crust has received a very considerable amount 
of attention. Perhaps one of the most fruitful limes of inquiry 
has been that which has aimed at determining the connexion 
between the varying densities of the crustal rocks and the different 
degrees of relief of the surface of the earth. It soon became apparent 
that in mountainous or elevated regions the deep-seated rocks were 
