Reviews—Prof. Joly’s Radio-activity and Geology. 177 
less than 70 tons of sedimentary rock” (p. 48). Actually, of course, 
more than 100 tons of sedimentary rock are produced, because most 
of the materials carried in solution to the ocean are afterwards 
incorporated by sediments, and, moreover, a good deal of material 
is abstracted from the atmosphere during the process of weathering. 
However, since the same meaning is consistently assigned to 
‘‘sedimentary rocks” throughout the calculations, no error is thus 
introduced in the time estimates. 
The amount of sodium annually contributed to the ocean is given 
as 175 million tons. This figure implies that the sediments now 
undergoing solvent denudation must lose all the sodium they contain, 
for the average amount of sodium in sediments, as exposed on the 
lands, is only 0°85 per cent. Unless, indeed, the igneous and other 
crystalline rocks of the earth’s surface—and these do not cover 
more than a quarter of the land areas—also lose all their sodium 
(and we know they do not), then the sediments must lose more 
than they appear to have originally contained. ‘This remarkable 
discrepancy shows either that the statistics of denudation are wrong, 
or that solvent denudation is now far more active than it has been 
on an average in the past, or that sodium is in some way restored to 
the sediments from sea-water. The latter possibility, exemplified by 
wind-borne salt, is further suggested as probable by the presence 
of salt waters in deep mines, and by phenomena of adsorption, 
whereby oceanic salts would become concentrated and trapped in 
sediments while they yet lay on the sea-floor. That denudation 
may be unusually active at the present time is a view that 
Professor Joly does not support. In favour of his contention to 
the contrary, he quotes figures to show that Europe, with the 
lowest average elevation among the continents, is undergoing the 
most rapid denudation. However, it is just in Europe that 
agricultural pursuits have been longest followed, and who can 
doubt that agricultural processes may enormously add to the ease 
with which the natural agents of denudation can attack the land. 
In ‘the case of the other continents there does appear to be a 
relation between mean elevation and total denudation. In so far, 
therefore, as the continents and mountain ranges stand higher 
to-day than they have averaged in the past, we may expect the 
intensity of denudation to be correspondingly increased. The 
remaining possibility, that the statistics of denudation are wrong, 
only enhances the difficulty, for when the United States data are 
_ added to those already available, the amount of detritus is found 
to be less than Joly gives, while the amount of sodium remains 
practically the same. It is possible, however, that owing to the 
difficulty of estimating sodium in river waters, errors may arise 
from inaccuracies of analysis. 
If the geological methods of measuring time as interpreted by 
Professor Joly are reliable, then it is necessary to show in what way 
the radio-active estimates are wrong. The latter give for the earliest 
known igneous rocks an age of about 1,500 million years. The 
pleochroic halo method gives a most probable value of 400 million 
years for the Leinster Granite (late Silurian or early Devonian). 
DECADE VI.—VOL. III.—NO. Iy. 12 
