
ALTERATION AND METAMORPHISM 225 
argillaceous strata, but may affect rocks of the most diverse 
character, as greywacké, conglomerate, and crystalline erup- 
tives ; but in such rocks it is never well developed, the planes 
of cleavage being usually imperfect and more or less irregular 
and discontinuous. 
Although the clastic character of ordinary clay-slate is 
sufficiently obvious, the rock is nevertheless not quite devoid 
of all crystalline structure. Now and again the surfaces of 
the cleavage-planes show scales of mica and needles of rutile, 
and such indications of incipient metamorphism gradually 
increase as the centre or axial zone of a much disturbed 
region is approached, with the result that clay-slate merges 
into phyllite. Followed in the same direction, phyllite in its 
turn passes into mica-schist, while the foliation of the latter 
may become more and more puckered and crumpled as the 
contortion of the rocks increases. Finally, the mica-schist 
may merge into a gneiss. 
The changes involved in the passage of a clay-rock through the 
several stages of slate, phyllite, schist, and gneiss are obviously partly 
mechanical, partly chemical. No doubt the rocks undergoing deforma- 
tion would be more or less deeply buried, and subject therefore to the 
pressure of overlying masses and possibly also to the action of the 
internal heat of the earth. However that might be, it is obvious that 
while the process of flexing and folding was going on, heat would 
necessarily be evolved and continue to augment as compression in- 
creased. The ingredients of the rocks would be mechanically crushed 
and flattened, while at the same time chemical action would be stimu- 
lated, and in the presence of water recombinations of the rock-materials 
would be effected—the minerals thus formed being arranged with their 
longer axes parallel to the planes of compression. Clay-rocks, com- 
posed as they are of fine-grained and relatively soft ingredients, would 
naturally offer least resistance to compression—cleavage-structure would, 
in their case, be readily superinduced. On the other hand, coarse- 
grained rocks, whether clastic or crystalline, would not be so readily 
affected. Their constituents being individually larger and usually more 
resistant than those of an argillaceous rock, a greater degree of pressure 
would be required to crush and flatten them. Hence it is that coarse- 
grained beds interstratified with clay-slates often show little or no trace 
of change beyond mere induration. When such coarse-grained rocks, 
however, are followed towards the zone or centre of greatest disturbance, 
they all eventually yield and become cataclastic. The rounded stones of 
a conglomerate and the angular fragments of a breccia, for example, are 
crushed, flattened, and elongated until they appear as mere lenticular 
streaks or cease to be recognisable, stones and matrix together being 
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