418 NATURAL SCIENCE [December - 
been to the petrologist. Each is regarded as an unmitigated nuisance, 
interfering with the observation of facts for which the enquirer is. 
searching; and as the stratigrapher is expected to map solid rock. 
through its drift covering, the petrological specialist, when consulted 
as to the character of a rock, is expected to say, not what it is, but 
what it may once have been. 
Professor Merrill, however, has endeavoured, in the work under 
review, to make the processes and results of weathering as interesting 
and full of real importance as the American and English glacialists. 
are making the drift. For it is the weathering of rocks which forms 
soil, the link between the dead earth-crust and the living plants and 
animals upon it. Geologists have studied rocks in all their various. 
kinds, their origin, differentiation, and metamorphism, their birth, 
growth, and life ; we have now to study their death, and the earliest 
parts of the process which culminates in their resurrection. 
In order to make his book self-contained, Professor Merrill devotes: 
several chapters to considering the chemical and mineralogical com- 
position structures, mode of occurrence, and various types of rocks ; 
this is a fairly useful summary, but we are inclined to doubt whether 
the geologist will need, or the lay-reader understand it. Many of the 
illustrations in this part are admirable, and the abundant analyses are- 
particularly valuable for reference later on. ‘To many of these no 
references are appended, and we may conclude that they are due to- 
the author himself, who must have expended a great deal of time and 
labour on the analyses throughout the work. 
Part III. is devoted to the weathering of rocks, each of the 
chemical and mechanical agencies being taken in turn. Several 
valuable instances of the effect of alternations of high and low 
temperature are given, and the effect of cold rain on highly heated 
surfaces is referred to. While stress is laid on the effect of hydra- 
tion, the work of carbonated rain-water, and of the humic, ulmic,. 
and crenic acids naturally comes in for a lion’s share of consideration. 
The action of the first of these solvents upon many silicates can be de- 
tected within ten minutes, while forty-eight hours’ digestion will obtain. 
from some amphiboles, epidotes, felspars, etc., quantities of lime, mag-. 
nesia, iron, alumina and silica, amounting to from 0-4 to 1 per cent. 
of the mass. Hornblende is more easily acted upon than felspar, and 
even magnesian silicates are attacked, so that serpentine cannot be 
considered a final product of decomposition. Increasing the pressure 
on the solvent has much more effect than prolonging the time of its 
action. Daubrée’s experiments on attrition are referred to, and the 
work of plants, bacteria, termites, and marine animals on the sea-bed,, 
is not neglected. 
Special cases of weathering are next treated in detail and illus- 
trated by full analyses. Mere bulk analyses of the fresh and; 
weathered rock are misleading, as they do not show all that has 
actually occurred. It is necessary to ascertain which constituents 
are least liable to be leached out, and to recalculate the analyses on 
the assumption that they remain constant. Alumina and iron oxides. 
are least liable to this, and the analyses of acid rocks are worked out 
on this assumption. An example will show the value of this method; 
the one chosen illustrates, in addition, that while only 30 per cent. of 
