322 Reviews—Lapworth and Page's Geology. 
Hydro-, Pyro-, and Dynamo-Metamorphism are used ; the first stages 
of their action producing altered, but the higher stages metamorphic 
rocks. Schists are defined as Foliated: crystalline sas, 3 in which the 
“ folia are not in parallel sheets but thin lenticular plates, each plate 
thickening in the middle,” ete. We have next descriptions of Augen 
structure, flaser structure (formed of lenticles or ellipsoidal patches 
or phacoids in a fine base), granulitic and mylonitic structures, the 
latter being defined as “Compact slaty-looking rocks, composed of 
material ground to powder, or rock flour, between the great moving 
masses in the over-faults of that region, like corn between a pair of 
millstones.” The author takes the responsibility of such of these 
terms as are due to him, and whatever may be the fate of the terms, 
the origin of the structures as made out in the Highlands seems to 
be almost undisputed. By Messrs. Blackwood’s kindness we are 
enabled to reproduce the figures which accompany these descriptions. 
Then follows an account of the two main sets of opinions on the 
origin of these rocks, leading to a description of the Highland section. 
The picture of this is almost an exact copy of Murchison’s, teaching 
us that to the mere drawing of sections must now be added the 
skilled interpretation of them. We then have a highly condensed, 
but very valuable account of the Deformation Theory of Metamorphism 
which we print in full :— 
“Founded upon the conclusions drawn from all these discoveries, 
a new theory of the cause of the association and of the varied petro- 
logical characters of the rocks ina district of regional metamorphism 
is being gradually developed at the present time—a theory most 
closely related to the original suggestions of Darwin, Scrope, and 
Sharpe. According to these new views, the rocks in such a district 
do not necessarily belong to any one distinctive geological period, 
but may be of various geological ages, owing their present association 
to the effects of lateral pressure, which has more or less obscured the 
evidences of their original relationships. Some metamorphic areas 
may possibly be wholly Archean ; others may be composed of patches 
originally either Archean, post-Archean, plutonic, sedimentary, or 
volcanic. The present structures of the rocks in various parts of 
such an area may be either original or secondary, according to the 
mode or degree of the local metamorphism to which they have been 
subjected. The granites, gabbros, etc., in such a region are unaltered 
plutonic rocks; the quartzites, crystalline limestones, and the like, 
are probably sediments only partly altered. The gneissoid rocks 
may be either Archean or post-Archzean plutonic rocks, or felspathic 
sediments foliated by pressure, intrusion, veining, and the like. The 
schists may be either metamorphosed sediments, retaining locally 
their original bedding (where altered by pyrometamorphism and the 
like), in dynamo- metamor phic rocks showing only secondary struc- 
tures, but which may have been originally either eneissic, plutonic, 
or sedimentary rocks, but whose ingredients have ‘een more or less 
completely rearranged both structurally and mineralogically. In 
fine, a metamorphic. area is a petrological complex whose altered rocks 
have a common foliation. The strike of foliation in such a district is 
