22 PROF. T. 6. BONNEY ON THE GEOLOGY 
the change as resulting from an action which has affected both 
masses to a different extent—there is another which to my mind is 
conclusive. All the rocks of this region—slates as well as schists— 
have been greatly folded and bear unmistakable indications of 
lateral compression. In those which some observers might call phyl- 
lites, those which, as described, at first sight resemble schists, the 
aspect of the rock leads us irresistibly to the conciusion that to this 
compression is due such modification in mineral structure as they 
have undergone; but the contrary is the case with the schists, and 
especially the mica-schists. Folded and corrugated schists are far 
from uncommon in nature. In some cases the structure of the 
crystalline constituents indicates that the folding has been the first 
stage in the process of metamorphism, and that the chemical or 
mineral change has been subsequent to the mechanical one. In others 
the former appears in immediate connexion with the latter, subse- 
quent to it, indeed, but in such connexion that the mineral consti- 
tuents, owing to the plasticity of the mass, were capable of change 
of position mutually without visible rupture of continuity or sign 
of crushing; but in a third case it is obvious that the rock, when 
compressed, was substantially the same that itis now. Im the first 
and second cases we find that the rock does not separate along the 
different mineral layers, where corrugated, more readily than where 
these are level; pieces may be struck off from it upon whose broader 
faces sections of many corrugations of the mineral layers are exhi- 
bited. Not so, however, when the lateral compression has been long 
subsequent to the metamorphism. Then not only do we find eyi- 
dence of brecciation in the lamine and veins of quartz &c., but 
also the arched bands of different minerals fall readily apart, and 
the rock, when hammered, demeans itself in a wholly different 
manner. 
We also meet with abundant confirmatory evidence of the above 
view in examining the microscopic structure of the rock. The mica- 
layers have evidently been bent into zigzags subsequently to their 
crystallization. Sometimes they have gaped, and the aperture has 
been filled by infiltrated quartz or other minerals ; sometimes there 
is distinct brecciation of the mineral constituents and the lke*. 
Now in the case of these metamorphic rocks of South Devon—the 
chloritic rock, with its frequent clastic aspect, especially as regards 
its quartz, the mica-schist with its wrinkled surface, and, especially 
at the Bolt Head, its peculiar fracture into prisms with corrugated 
sides, and its wonderful microscopic structure—there is, to my mind, 
every sign that the metamorphism and the great lateral compression 
of the rocks were wholly distinct, and the latter long subsequent to 
the former; and I believe that any petrologist, accustomed both to 
handle rocks in the field and to examine them with the microscope, 
will be led irreststibly to the same conclusion. 
* TI only mention two or three of the most obvious signs. There are others 
familiar to me which I do not mention, because they would require too long a 
digression, in order to convince any but the very few who have made a special 
study of these questions. 
