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222 _ STRUCTURAL AND FIELD GEOLOGY 
are now exposed ; but although those lower beds must have been subject 
to the action of plutonic heat, they yet remain unaltered. On the other 
hand, much younger formations, which have not been concealed under 
any considerable thickness of rock, have nevertheless in some cases been 
highly metamorphosed. 
Certain recent observations in Finland, by Dr J. J. Sederholm, would 
seem to show, however, that the old Huttonian view, as subsequently 
modified, may have greater significance than many geologists have 
recognised. Dr Sederholm sets forth certain remarkable evidence 
which has led him to conclude that wholesale “refusion or resolution ” 
of certain pre-Cambrian rocks (consisting of granitoid gneisses with 
subordinate sedimentary strata) has actually taken place. According 
to him this melting process must have been effected at a time when 
these rocks were buried under a great thickness of rock-masses, removed 
since by denudation. The pre-Cambrian strata are believed to have 
been so deeply depressed that they approached the highly heated interior 
or “bottomless magma ocean” of the earth. Under such conditions, 
the rocks in question appear to have been largely melted or resorbed 
by the magma, and thus eventually transformed into crystalline granitoid 
masses. Through these are dispersed isolated fragments (xenoliths) of 
the original rocks which are often fused to such an extent that they are 
almost effaced. 
(b) Hydrochemical Metamorphism.—In opposition to the views upheld 
by the supporters of plutonic metamorphism, Bischoff, in his famous 
work (Chemical and Physical Geology), maintained that high temperature 
and pressure were not required to account for the phenomena of the 
crystalline schists. He showed that water slowly percolating through 
the rocks would act as a reagent—breaking up minerals and inducing 
multitudinous recombinations, and that all the constituent minerals of 
schistose rocks could be produced in the wet way at ordinary tempera- 
tures. His conclusions were largely based on the study of pseudomorphs, 
which he had no difficulty in showing frequently occurred in rocks 
that gave no evidence of having been subjected to heat. One mineral 
could be altered into another either by the loss or the gain of an ingredient, 
or by the exchange of ingredients. Or there might be a total change 
of substance—the new mineral containing none of the ingredients of its 
predecessor. If this could be the case with crystallised minerals, 
similar changes must affect sedimentary rocks—out of clay, for example, 
all the minerals of gneiss might be developed by chemical reactions. 
The hydro-chemical theory is thus plausible enough, and explains many 
of the alterations which all rocks have undergone. Bischoff’s work was 
of essential service, and must still be studied by geologists who are 
interested in the remarkable transformations which are brought about 
by the action of meteoric water making its way down from the surface. 
The theory fails, however, to account for regional metamorphism. If 
it were well founded, then all the oldest sedimentary formations should 
long ago have been metamorphosed, while the younger systems should 
never show any trace of such change. Yet we find that in many places 
