Reviews—A. Daubrée’s Experimental Geology. 135 
water, in horizontal cylinders of iron or earthenware. Their mutual 
friction soon reduced them to pebbles and fine mud. Sand is formed, 
but in subordinate quantity, and is usually angular; rounded only in 
exceptional cases. Quartz-sand, if derived from granite, is, M. 
Daubrée says, always angular. Hach sand bears in some sort the 
_ Stamp of its source and cause, and provides material for the study of 
former physical conditions. A section of this chapter discusses the 
distribution of gold in the bed of the Rhine. 
The interesting chemical actions which presented themselves in 
these experiments were commented on in the review of the first 
section of this work. 
The chapter concludes with an account of experiments on the 
striation of rocks. They were conducted by rubbing pebbles of 
quartz and felspar on plates of granite, with various pressures and 
velocities. By this means strie could be perfectly imitated, and it 
appeared that neither high velocity nor high pressure were indis- 
pensable. Indeed, the velocity required for the commencement of 
striation seems to vary inversely as the pressure acting; so that 
striation depends on the product of the measures of pressure and 
velocity. An interesting observation is that under sufficient pressure 
softer rocks may striate harder. 
The pebbles must be firmly fixed in order to engrave the plate of 
granite ; if imbedded in a plastic mass such as clay, they slide and 
fail to scratch. 
The second chapter, incomparably the most important and in- 
teresting, treats of the various deformations which the earth’s crust 
has experienced, and the infinitely numerous fractures which sub- 
divide it. These in their vast dimensions, their unlimited variety, 
might be expected to mock our feeble attempts at imitation of them. 
But Sir James Hall’s classic experiments on the contortion of strata 
have commenced the work of explanation, and M. Daubrée, follow- 
ing in the same track, has provided a mass of materials for 
controversy. The first section of the chapter is devoted to folds 
and contortions, which are imitated by layers of metals, wax, etc., 
subjected to fixed vertical pressures, and acted on by gradually in- 
creasing longitudinal forces. If the vertical pressure be uniform, the 
folds produced are regular and symmetrical; but when the vertical 
pressures are unequal, the folding also ceases to be symmetrical. 
The contortions are most acute where the superincumbent pressure is 
least, as might be easily conjectured. Interesting illustrations show 
the growth of reversed folds, such as are well known in the Alpine 
Chain and elsewhere. 
He then proceeds to discuss the nature and causes of faults, joints, 
and generally all kinds of fractures of the sedimentary rocks. Here 
we enter the battle-field of an ancient controversy, long waged with 
no decisive result, and perhaps even the powerful reinforcements 
which M. Daubrée has brought up may not be sufficient to secure 
a victory. If the origin of faults be not perfectly known, still few 
will now attribute them either to molecular actions, or to shifts in the 
position of the earth’s equator—opinions which are alluded to as once 
