.? 
~ 
582 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Book IV. 
been effected with little or no alteration of the ultimate chemical 
composition of the mass. In other cases a perceptible alteration in 
the proportions of the chemical ingredients is traceable.1 The 
development of a crystalline structure can be traced through inter- 
mediate stages from ordinary sedimentary rock to thoroughly foliated 
schist, remains of fossils being still observable after considerable 
progress has been made towards the completion of a crystalline 
rearrangement. 
6. Not only does the crystalline character increase towards the 
limit of contact with the eruptive rock, but it is accompanied with a 
progressive development of foliation, the minerals, more especially 
the mica, crystallizing in folia parallel either with the original 
stratification of the clastic mass or with the cleavage surfaces should 
these be its dominant divisional planes. Along the line of contact 
with granite the foliation is sometimes excessively crumpled or 
puckered, while here and there the foliated structure disappears and 
the rock assumes a lithological character closely approximating to 
that of granite. 
7. The phenomena now described evidently point to the heat of 
eruptive rock as their prime cause. Mere dry heat, however, would 
probably have been ineffective for the production of the changes 
observed. It was accompanied by the co-operation of water, either 
already present interstitially in the sedimentary rocks or supplied to 
them from the eruptive masses. From experimental researches it is 
known that at a dull red heat in presence of water, important 
mineralogical transformations take place (ante, p. 800). There is 
reason to believe that by a reaction of this nature the phenomena of 
contact metamorphism were produced. 
§ IIlI.—Regional (Normal) Metamorphism. 
From the phenomena of metamorphism round a central boss of 
eruptive rock we now pass to the consideration of cases where the 
metamorphism has affected wide areas without visible relation to 
eruptive matter. It is clear that only those examples are here 
admissible in evidence where there is distinct proof that the 
crystalline and foliated character passes into that of ordinary stratified 
materials, or where the rocks can be shown to be the equivalents of 
what are elsewhere ordinary unaltered masses. 
At the outset it must be observed that a feeble but distinct 
trace of metamorphism is indicated by abundant veins of quartz and 
calcite which tell of a copious penetration by water charged with 
1 This is specially noticeable in the proportion of silica, which is sometimes found to 
be largely increased in the altered zone, either by an absolute addition of this acid, or 
by solution and removal of some of the bases. See Kayser, Z. Deutsch. Geol. Ges. xxii. 
p. 153. ful 
2 In the south of Scotland the foliation round the granite bosses is coincident with 
stratification ; round Skiddaw, with cleavage. 
