Reviews—A. Daubrée’s Experimental Geology. 137 
visible displacement, just as in the production of columnar structure 
in igneous rocks, the contraction which has relieved the tension is 
often almost microscopic. The regularity and smoothness of joints 
agrees much better with pressure than with tension. The shearing 
stress of a pressure requires a plane face of rupture to permit the 
sliding which will relieve it, while a tension could be eased and 
ended by the opening of any crack. however irregular. It might 
also be alleged that contraction ought to produce hexagonal columns, 
or rhombohedrons and spheroids, while pressure, as in these experi- 
ments, developes rectangular blocks. However, Jukes mentions 
cuboidal jointing developed in certain cooled slags, which is clearly 
a consequence of contraction. 
The sections devoted to the connexion of valley-systems with 
faults and jointing will scarcely satisfy physical geologists. But 
the maps and diagrams given are very interesting. The maps are 
ingeniously accompanied by superposed transparent diagrams of the 
general lines of water-flow. These do show a marked tendency to 
arrangement in two systems mutually at right-angles. Some evi- 
dence is given that there are systems of faults and joints parallel to 
these, but the proofs offered that certain valleys could not have been 
formed by erosion. without joints or faults to guide it, will hardly be 
considered conclusive. There is no allusion to the theory of rivers 
cutting their way across ridges in process of elevation. The canons 
of Colorado are quoted as scarcely opened fissures. 
M. Daubrée introduces the terms Paraclases to denote fractures 
with a throw; Diaclases for simple surfaces of separation ; Litho- 
clases, to include both. The latter should surely be Petroclases, 
and there appears no distinction between the former terms and 
faults and joints respectively. Is it not a pity to coin new words 
unnecessarily ? 
Some experiments are described on the origin of indented pebbles, 
a phenomenon to which we remember no reference in English books, 
and some pretty figures are given of the mutnal actions of a non- 
contractile envelope and a contracting nucleus. Systems of wrinkles 
are the result, answering to mountain chains on the surface of the 
earth. 
While in the second chapter M. Daubrée treats of a controversy 
still active, in the third he takes up one generally regarded as set at 
rest. Sharpe, Darwin, Sorby, and Tyndall, have proved that slaty 
cleavage is due to lateral pressure. It is, however, not universally 
admitted that foliation is due to the same cause. M. Daubrée seems 
to consider them not only due to the same cause, but to be the same 
thing, and applies the single term “schistosité”’ to them both. It is 
well known that pressure has produced a laminated structure in 
some igneous rocks, and M. Daubrée’s remarks on the production of 
foliated granite in a similar manner are well worthy of consideration. 
Instead of the fashionable belief in the metamorphism of sedimen- 
tary rocks into granite, he here appears to maintain the possible con- 
version of granite into gneiss. 
_ But when M. Daubrée seeks to illustrate schistose structure by 
