

226 STRUCTURAL AND FIELD GEOLOGY 
converted into a mylonite or into a crystalline schistose aggregate. 
Granitoid crystalline rocks are in like manner crushed down, recrystal- 
lised, and foliated. Not infrequently, in such crushed eruptives, 
lenticular cores (or Dhacoids, as they are termed) of the original rock can 
still be observed, around which the finely pulverised and recrystallised 
materials are arranged much in the same way as the smaller crystalline 
ingredients of a lava have grouped themselves about a phenocryst. All 
the phenomena, in short, conspire to show that the metamorphosed 
rocks in question have been so compressed and crushed that they have 
been compelled to flow. 
Just as in thermal or contact metamorphism the rocks become in- 
creasingly affected as a plutonic mass is approached, so also in regional 
metamorphism we encounter gradually augmenting rock-changes while 
we proceed from the peripheral areas of comparatively unaltered rocks 
to the entirely reconstituted masses of the interior region. Advancing 
towards the latter region we first encounter, it may be, slates, phyllites, 
hydrous mica-schists, chlorite-schists, serpentine, diabase-schist, green 
schists, conglomerate-schists, and other rocks similarly indicative of less 
intense metamorphic action. Next we enter a region, the most 
characteristic rocks of which are andalusite-, kyanite-, and staurolite- 
schists, mica- (muscovite, biotite) schists, amphibole-schists and 
amphibole-rock, granulite, gneisses (usually fine-grained), etc. Reach- 
ing the inner zone, we are confronted with coarse biotite-schist, fre- 
quently containing garnets, granulite, jeclogite, biotite-garnet-gneisses 
(often coarse-grained), hornblende-gneiss, amphibolite, etc. Quartzite, 
crystalline limestone, and calc-mica-schist may be present on any 
horizon. 
Here, then, we have much the same succession of changes as are 
supposed to have occurred in the case of plutonic metamorphism. And 
upholders of the latter theory would probably claim such a succession as 
favouring their own view. The present folded and crumpled aspect of 
the schists, they might say, were the result of subsequent crustal 
deformation. 
The theory of dynamo-metamorphism explains so many striking 
phenomena which are hard to account for by the plutonic theory that it 
is accepted by many geologists as giving a reasonable interpretation of 
regional metamorphism as a whole. Nevertheless there are difficulties 
in the way of accepting it as generally applicable. For example, in 
many places the highly convoluted strata of certain mountains of uplift 
show no evidence of true metamorphism—the petrographical character 
of clay-slates, greywackés, sandstones, and limestones has remained 
unaffected during the process of compressing and folding. Again, there 
are regions where highly crystalline schists occupy undisturbed positions 
—that is to say, they are not plicated or folded. Once more, it has been 
shown that the process of metamorphism has in some cases preceded 
that of rock-folding. 
The probabilities are that metamorphism is the result sometimes of 
contact with batholiths or even with the heated interior of the earth, and 


