C.—GEOLOGY. 69 
pulsations. ‘The later revolutions have been less profound than the 
great convulsions of the Archean, but diastrophism may make up for this 
by becoming more recurrent, tending to stimulate in post-Palzozoic eras 
the mean rate of erosion and sedimentation.’ 
The general conclusions seem to be that the earth is not showing a 
trend towards rigidity, but that earth movements and vulcanism are 
becoming less profound in scope and less widespread geographically, the 
average of activity being maintained by more frequent recurrence. 
2 Earth Movements and the Nicety of Adjustment. 
The causes of earth crumpling and the dynamic laws which govern 
_ the phenomena are subjects well within my theme, but their consideration 
_ would lead us to undue length. Earth movements, of necessity, are 
bound up with theoretical considerations of the earth’s interior. Whether 
earth crumpling is due to loss of terrestrial heat and consequent contraction 
of the nucleus, or whether the modern concept of isostasy offers a better 
explanation, there must be a downward limit to terrestrial disturbances. 
This limit has been placed at 113°7 kilometres and termed the ‘ depth of 
compensation ’. 
According to the principles of isostasy great blocks of the upper crust 
float higher or lower above the depth of compensation according to the 
specific gravity of the mass involved. This is a grand conception and 
offers much food for thought. If isostasy is the true explanation of moun- 
tains and oceanic depths, it follows that the calculated gravity should be 
uniform over the earth’s surface if all parts of that surface are in isostatic 
equilibrium. Observations do not entirely support the conclusion, as 
many anomalies have been observed. The subject is attractive and the 
literature extensive. In reviewing the results of 325 gravity determina- 
tions in the United States, Dr. David White explains the observed 
anomalies as due to local causes beneath the gravity stations. ‘ Hence 
the equilibrium of the crust beneath the gravity stations is very much 
nearer complete than is indicated by the anomalies as uncorrected for 
local abnormal densities relatively close to the instrument.’ (Presidential 
Address, Geological Society of America, 1923.) 
lt is obvious that a consideration of this subject would lead to a dis- 
cussion of land forms and their influence on human activities—definition 
of nationality, physiographic control, distribution of faunas, and countless 
other effects, all of which extend beyond the realm of technical geology 
and form part of a general education if they are not ‘cultural’ in the 
narrower sense. 
There is, however, one great lesson to be derived from the study of 
earth movements that bears on the general scheme of things and is worthy 
_of especial mention—the marvellous continuity of conditions. 
The diameter of the earth is about 8000 miles and the maximum of 
_ relief of the lithosphere about eleven miles, approximately 0°14 per cent. 
_ Oceanic waters have filled the depressions and continental masses have 
_ risen above the water-level—a condition that has maintained throughout 
_ all time in the opinion of most geologists. The present area of the land 
is 27-7 per cent. of the whole terrestrial surface, and the average height 
above sea-level of the continental masses is only 2120 feet according to 
de Lapparent. It is apparent that the actual volume of that part of the 
