314 SECTIONAL TRANSACTIONS.—C. 
The reappearance of many Barton species in the Middle Headon Beds of the 
Oligocene is deserving of careful attention. This apparent blending of Eocene and 
Oligocene marine species is all the more striking because a substantial thickness of 
freshwater beds intervenes between the Upper Barton and the Middle Headon Beds. 
10. Dr. C. A. Mattey.—An Armoured Dinosaur from Southern India. 
About 600 or 700 scutes, together with a few spines and some small bones, of 
a Stegosaurian were recently discovered by the author in the Utatur Beds of Southern 
India. The only other record of an armoured dinosaur from India is the type specimen 
of Lametasaurus indicus, Matley, from the Lameta Beds of Jubbulpore. 
AFTERNOON. 
Excursion to Bournemouth. Leader, Mr. Henry Bury. 
Saturday, August 29. 
Excursion to the Isle of Wight. Leader, Mr. G. W. Colenutt. 
Sunday, August 30. 
Excursion to Lulworth Cove. Leader, Mr. T. F. Grimsdale. 
Monday, August 31. 
Mornine. 
11. Joint Discussion with Section A on Variation in Gravitational 
Force and Direction and its relation to Geological History. 
Dr. J. W. Evans, F.R.S.—Isostasy and Geological Structure. 
Isostasy is the expression of the fact that there is an almost complete balance of 
different portions of the earth’s surface. It implies that the total mass underlying 
a square mile of the earth’s surface, whether it is in the Himalayas or under the ocean, is 
practically the same. Any disturbance of this equilibrium by external changes 
such as the accumulation of sediments, ice sheets, lavas or volcanic ashes, or the 
removal of material by erosion, melting, or other means, is set right in consequence 
of the inherent mobility of the substance of the earth. 
Purely local deviations from isostasy, a few hundred square metres in extent, may, 
it is true, be maintained indefinitely by the inherent strength of the rocks, and those 
of somewhat greater area may be adjusted so slowly that it will be many centuries 
before the process is complete, but on a large scale isostasy is practically perfect. 
How then is the adjustment effected 2? According to one school, mainly composed 
of geodesists, it is the expansion of the rocks below that brings mountains and plateaux 
into existence, and their contraction that gives rise to the ocean depths. Others, 
mostly geologists, explain the inequalities of the surface by the operation of simple 
and recognised geological processes and structures. The continents, they believe, 
are made up of lighter rocks, the ‘sial,’ floating in heavier plastic materials, the 
‘sima.’ When the sial is crumpled into folds by lateral compression, these would 
be piled up into mountains many times loftier than those which they now form, if 
it were not that as fast as the fclding accumulates additional material at any point, 
the folds sink into and displace the sima below, until the isostatic balance is obtained. 
Then, as the erosion of the mountains proceeds, the folds rise once more in order to 
restore isostatic adjustment. 
The adherents of the former school assume that the additional mass of the moun- 
tains or other elevations is compensated by the existence below them of a column 
of uniform density, less than the mean, and that this uniform column extends down 
to a level of equilibrium, where the pressure is the same all over. the world; and that 
in the same manner the defect of density in the oceans is compensated by a column of 
uniform density greater than the mean, and that this also extends down to the same 
level of equilibrium. 
