Reviews—Lapworth and Page’s Geology. 320 
related to the general strike of the sheets of sedimentary or crystalline 
rocks which formed its main masses at the time of its final folding 
and shearing; the dip of foliation is simply the local direction of 
shear,—i.e. is related to the direction in which the mass to which it 
belonged was giving way. Like cleavage, it may locally coincide 
with the original bedding ; but it normally crosses the bedding at an 
angle, and traverses aqueous and igneous rocks alike. The planes of 
schistosity are those planes along which the rocks yielded to the 
lateral pressure or torsion, and these yielding planes may be of all 
grades of importance, from the great overfaults, along which solid 
masses of enormous extent were thrust forward, in some cases for 
scores of miles, down to the minutest planes separating the micro- 
scopic folia of the slaty schists. These planes cut the rock up into 
lenticular patches or ‘phacoids,’ which, like the yielding planes 
themselves, are of all gradations of size—from the mountain masses 
riding out along the great overfolds and overfaults, down to the 
‘eyes’ of the ‘augen’ schists. The mechanical effects wrought by 
the shearing and deformation of the phacoids show of necessity a 
corresponding gradation, the conditions at one extreme giving rise to 
coarse rock-breccias ; in medium cases to flaser structures and foliation ; 
at the other extreme to the compact, stringy mylonite, in which the 
original material has been torn and ground to rock-dust. Parallel 
with these mechanical changes, but not necessarily accompanying 
them, we note a series of chemical changes of rising grades of impor- 
tance—a larger and larger portion of the rock undergoing deforma- 
tion becoming recrystallized, until finally, it may all become trans- 
formed into foliated crystalline rock. The maximum of mechanical 
effects seem to have been wrought where the rock yielded to the 
excessive pressure and torsion mainly along certain definite planes 
(shear planes or gliding planes) or areas within the mass; the maai- 
mum chemical effects where the deforming stresses affected all the 
particles of the mass alike, the rock yielding or flowing in the manner 
of a plastic or liquid body. In some minor areas the results effected 
by dynamo-metamorphism possibly approximate to those wrought by 
pyro-metamorphism, and we may have what has been called stratifi- 
cation foliation. In general, however, the foliated rocks have 
been sheared, and the primary structures have all been more or less 
obliterated. But, as a metamorphic region may be subjected to suc- 
cessive earth-movements acting at different times, and from different 
directions, we occasionally find a newer and more or less incomplete 
foliation crossing an older foliation, or the rocks may show traces of 
a foliation ofa still earlier date: the successive foliation planes being 
in different stages of development, preservation, or obliteration.” 
So much has been done in so little space that it seems greedy to 
ask for more; still, in the next edition we hope the author may be 
able to supplant the rest of the old work which he has left ; to give 
us a few more figures of type sections, a little more space to the 
dates and function of earth-movements, and even perhaps also to the 
physical geography of the different periods. It seems unnecessary 
to put into words what it is quite certain all readers will think of 
