
ALTERATION AND METAMORPHISM 223 
the very oldest fossiliferous strata (Cambrian), although they must have 
been subject to the action of percolating water for untold millions of 
years, are nevertheless quite unchanged. On the other hand, strata 
belonging to relatively recent times (Tertiary) have in some places been 
rendered crystalline and schistose. Even if these contradictory facts 
could be reconciled or explained away, we should still be unable to 
explain the appearance presented by the schists themselves. These, as 
we have seen, are arranged in layers or beds of very different chemical 
and mineralogical constitution—mica-schist, for example, is found 
alternating with, but sharply marked off from, beds and layers of such 
rocks as hornblende-schist, talc-schist, gneiss, quartzite, serpentine, 
crystalline limestone, etc. Had the metamorphism of these rocks been 
caused by circulating water introducing and abstracting ingredients, as 
in the formation of pseudomorphs, there could have been no such 
arrangement of the schists as that referred to. The changes effected by 
percolating water would have been independent of bedding-planes, and 
would have been most in evidence along the more or less vertical joints 
and fissures by which the rocks are traversed. 
(c.) Dynamo-metamorphism.—N ew light was thrown upon the origin 
of regional metamorphism, when it became recognised that the altered 
rocks were usually somewhat highly folded, and that the intensity of the 
metamorphism was in direct proportion to the degree of crustal deforma- 
tion—crystalline texture and schistose structure becoming more and 
more pronounced as the centres or axes of greatest disturbance were 
approached. It was observed that in the areas of greatest disturbance 
highly crystalline and puckered schistose rocks predominated, and that 
as one passed outwards from such areas, rocks of that type gradually 
gave place to others in which crystalline texture and foliated structure 
became less and less prominent, and at last died away as flexing, folding, 
and rock-displacements continued to diminish in importance. 
The effects produced by this dynamo-metamorphism resemble in 
some respects those brought about by thermal metamorphism. In both 
cases alike, the changes have as a rule left unaltered the composition of 
the rocks attacked. Clastic rocks, owing to recombinations of their 
ingredients, have been rendered crystalline, while igneous and old 
schistose rocks have in like manner been recrystallised. In other 
respects, however, there are notable differences to be observed between 
the two kinds of metamorphism. In regional metamorphism, for 
example, we have no evidence of actual fusion, such as occurs now and 
again in the case of thermal or contact metamorphism. On the other 
hand, in contact metamorphism there is little to show that the altered 
rocks have been concurrently subjected to much lateral pressure, whereas 
the rocks throughout an area of dynamo-metamorphism give proof of 
having experienced the most intense compression. Again, in the case 
of contact metamorphism, foliation when present always coincides in 
direction with pre-existing planes of division, while in that of regional 
metamorphism such coincidence is more or less accidental, foliation 
having been developed usually along planes of compression. In steeply 


