438 Book Notice. 
is now introduced, 7.e., the amount of water in air-dried peat, while, on 
the other hand the error occasioned by adhering to the 100°C. basis would 
not be very appreciable, numerous other advantages of its use are obvious. 
(2) The other important point is a question of arithmetic, The ratio 
poms is quite incapable of yielding any measure of: the humus- 
content of peat. A few concrete examples will best illustrate this point, 
and for argument’s sake, suppose the peats have the following percentage 
compositions :— 
Humus 3 n= 2Q9°90 99:0 95'0 90:0 
Mineral oe ae ait I:0 50 10°0 
I00°0 I00'0 I00'0 I00'0 
Humus 
aGezal ae -. 999:0 99:0 19.0 9:0 
then, according to the ‘convenient method,’ the third line, which is 
obtained by dividing the first by the second, gives the humus content. 
Even a casual comparison of the first and third lines will suffice to establish 
the absurdity of the above equation. 
There is only one other point to which I desire to draw attention, 
and that is the exception taken to the statement that peat contains 
80 to go per cent. of moisture, and when air-dried 15 to 25 per cent. 
It is quite beyond the scope of the present note to enter into a discussion 
of the accuracy of such a well-known and generally accepted scientific 
fact, one can only add that reference to the most recent literature? fully 
justifies the original statement, and that the suggested changed standard 
of comparison is purely imaginary. 
J. W. H. JOHNSON, 
22nd August, 1913. Walton, Wakefield. 
3 OL; 
Igneous Rocks, Vol. Il., Description and Occurrence, by Joseph P. 
Iddings, 8vo. pp. xi.+ 685. New York: Wiley and Sons, 1913. This 
volume fully maintains the high standard of its predecessor, issued four 
years ago, and of “ Rock Minerals,’ now in its second edition ; and the work, 
as now completed, should be in the library of every serious student of 
petrology. In our notice of volume I. we confessed a curiosity as regards 
how the author would give effect to a systematic treatment of igneous 
rocks on the lines of the ‘ Quantitative Classification,’ to which he was 
committed. It would seem that trial has proved the impossibility 
of the task, for the scheme actually adopted follows, in its broad outlines, 
amore familiar plan. Asa first step, the rocks are ranged in six divisions, 
according as they are characterized by (1) preponderance of quartz ; 
(2) quartz and felspar; (3) felspar without quartz; (4) felspars and 
felspathoids ; (5) felspathoids, and (6) chiefly ferro-magnesian minerals. 
This is decidedly more acceptable than Persalane, Dosalane, and the rest. 
Group names, such as Syenite and Monzonite are used, but in many 
instances are redefined to introduce the element of numerical precision. 
The systematic description occupies the first half of the volume. The 
second half is a more original contribution, being an account of the 
geographical distribution of different kinds of igneous rocks over the globe. 
It brings together in connected form a large body of information, and will 
certainly be found very valuable for reference. The maps given in illus- 
tration of this part of the work would be more useful if the geological age, 
as well as the distribution, of the igneous rocks had been indicated.—A.H. 
1 ‘Dictionary of Applied Chemistry,’ by Sir Ed. Thorpe, page 606, 
Longmans, Green & Co., 1912. 
Naturalist, 
