176 Reviews—Prof. Joly’s Radio-activity and Geology. 
P.S.—Since the above was written Professor Swinnerton’s concluding 
paper on the classification of the Trilobites has been issued(Gnot. Mae., 
Dec. VI, Vol. II, p. 543, December, 1915). In it the three families 
Trinucleide, Raphiophoride, and Harpedide are placed in a separate 
' sub-order with three other families as a provisional appendix, and 
their line of descent is traced back to a Conocoryphid-lke stock. 
This view is in general accordance with my conclusion that Zrimucleus 
is more probably connected with the Opisthoparia than with the 
ill-established group Hypoparia. 
RAVLEWwSs- 
———<———— 
I.— Rapio-acrivity AND GEOLOGY. 
Tar Braru-time oF THE Wor LD, AND oTHER ScrenTiFic Hssays. By 
J. Jory, M.A., Sc.D., F.R.S. pp. 307. T. Fisher Unwin. 1915. 
10s. 6d. net. 
({VHIS volume is an eloquent witness to the love which its versatile 
author feels for all the activities of nature. It contains twelve 
essays, mostly written during the last few years, dealing not only 
with geological problems but also with such diverse subjects as the 
abundance of life, the colours of flowers, the ‘canals’ of Mars, 
the photographic image, the application of radium to medicine, 
and the physics of skating. These last are mentioned only to show 
the far-flung interests of Professor Joly and the broad appeal of his 
book; for it will be convenient to restrict the discussion in this 
place to those of the essays which are specifically geological: The 
Birth-time of the World, Denudation, Mountain Genesis, Alpine 
Structure, and Pleochroic Haloes. The problems considered fall 
approximately into two categories. First, the measurement of 
eeological time by denudational and radio-active processes, and second, 
the genesis of mountains. Pleochroic haloes, their explanation, and 
their application to determining the age of the minerals in which 
they occur, form the subject of a delightful essay which is already 
so well known that it calls for no comment here, but one of 
admiration. 
Some of the results obtained by Professor Joly for the age of the 
earth, based on geological methods, are as follows :— 
1. From the thickness of sediments . : 100-134 million years. 
oe a5 mass of sediments : : 87 i 75 
3. ua sodium in the ocean . : 99-105 ‘is +4 
It is unfortunate that the data used in arriving at an estimate of the 
detritus borne by the rivers to the sea do not include the recent 
results for the whole of the United States (Dole & Stabler, Water Sup. 
Pap., U.S.G.S., No. 284, 1910). Inthe light of this work the figure 
given for the annual increment of sediment—10,700 million tonnes, 
+ 11 per cent for bottom load—seems to be much too high. Here 
it may be mentioned that Professor Joly uses the term ‘‘ sedimentary 
rock’? where mechanically transported detritus is meant, as, for 
example, when he states that ‘‘ 100 tons of igneous rock yields rather 
