38  Reviews—Prof. J. Barrell—Isostasy. 
being the reference in literature where the description and figures 
are to be found, and afterwards are given the geological horizon, 
locality, and registration number. A valuable addition, we think, 
might have been introduced in connexion with the geological 
information offered, that of Continental equivalents, as for instance, 
when quoting the horizon of the Atherfield Beds as ‘ Lower Green- 
sand’ the term ‘ Barremian’ might have been given in brackets. 
A modification of this kind would have been most welcome to the 
foreign student of geology in enabling him at once to understand 
the value of a purely English term and its exact significance in the 
Cretaceous Series. It is interesting to mention that this catalogue 
differs from all the former, as it includes a number of type or 
figured specimens that were originally in the Museum of the Geo- 
logical Society of London, these being indicated by the letters G.S. 
EN. 
IX.—Isosrasy, AND THE ASTHENOSPHERE. 
Tur Srreneru or tHE Earrn’s Crust. By Josnpa Barrett. Journal 
of Geology, vol. xxii, pp. 28, 145, 209, 289, 441, 537, 655, and 
729, 1914; vol. xxiii, pp. 27, 424, and 499, 1915. 
N this remarkable series of papers, which is worthy of the most 
careful study, Professor Barrell discusses the problems arising 
from the existence, now well established, of isostasy. He shows 
from geodetic data and from geological evidence (e.g. the building 
up of extensive deltas) that ‘‘isostasy . . . is nearly perfect, is very 
imperfect, or even non-existent according to the size and relief of the 
area considered”. The close degree of isostatic equilibrium postulated 
by Hayford is not admitted, but since over large areas equilibrium 
is maintained in spite of denudation, deposition, and mountain- 
building, it follows that there must be some counter movement 
taking place within the earth. To explain the mechanism of isostatic 
compensation Barrell introduces the conception of an asthenosphere— 
a sphere of weakness—which, as he clearly shows, must be below the 
level of compensation, and of great thickness. The excess pressure of 
heavy columns of the lithosphere is transmitted to the asthenosphere, 
within which the lateral movements of restoration take place. In 
proof of the existence of some such zone of weakness the author 
indicates the impossibility of widespread flowage in the zone of 
compensation; he shows that below the level of compensation the 
earth is unable to withstand the stresses thrown upon it by the 
greater undulations of topographic relief; and finally he cites 
Schweydar’s conclusion, based on measurements of earth tides, that 
there seems to exist a yielding layer 600 kilometres in thickness 
below the 120 kilometres of the lithosphere. 
It is suggested that recrystallization is the chief factor determining 
weakness, plasticity, and flowage. It should be remembered that 
effective plasticity, signifying a low elastic limit, is by no means 
incompatible with a high rigidity. The flowage of glaciers is an 
instructive case in point. Recrystallization will clearly be favoured 
by high temperature conditions. This leads to another line of 
