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588 GHOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IY. 
unless we conceive the sediment to have been of the nature of the 
magnesian clays (sepiolites) of the Paris basin. It is possible, how- 
ever, that some of these magnesian rocks were originally of igneous 
origin, either erupted at the surface or intrusively injected among 
the surrounding rocks previous to metamorphism. Such mineral 
masses as varieties of syenite and diorite, rich in hornblende or other 
magnesian silicates, might have been the origin of many of the rocks 
here referred to. Fine schists consisting mainly of hydrous mag- 
nesian silicates may have been at first tuffs associated with the lava- 
form masses. 
§ I1V.—The Archean Crystalline Schists. 
We now finally advance to the consideration of those schistose 
rocks which underlie the oldest fossiliferous and sedimentary 
formations. On the whole they present the closest resemblance to 
tracts of regional metamorphosed rocks, though, as a rule, more 
coarsely crystalline, containing more massive bands of gneiss, horn- 
blende-rock, &c., and being more intricately veined with granite, 
pegmatite, and allied crystalline masses. ‘The most natural inference 
to be drawn as to their origin is obviously to regard them as derived 
from the metamorphism of ordinary sedimentary rocks. This con- 
clusion has been adopted by the majority of geologists. The Archean 
crystalline-schists are assumed to be of metamorphic origin, and 
indeed the phrase “ metamorphic rocks” is often used as a synonym 
for these oldest crystalline masses. But though their close resem- 
blance to the products of regional metamorphism may justify the 
inference usually drawn, it does not amount to a proof of absolute 
identity of origin. 
The difficulty of explaining some of the transformations which on 
the theory of metamorphism must have taken place, has led to 
another explanation. Some writers, justly repudiating the exagger- 
ated views of those who have sought by metamorphic (metasomatic) 
processes to derive the most utterly different rocks from each other 
(for example, limestone from gneiss and granite, granite and gneiss 
from limestone, tale from granite, &c.), have insisted that the 
cystalline schists, in common with many pyroxenic and hornblendic 
rocks (diabases, diorites, &c.), as well as masses in which serpentine, 
talc, chlorite, and epidote are prevailing minerals, have been de- 
posited “for the most part as chemically-formed sediments or 
precipitates, and that the subsequent changes have been simply 
molecular, or at most confined in certain cases to reactions between 
the mingled elements of the sediments, with the elimination of 
water and carbonic acid.” To support this view, it is necessary to 
suppose that the rocks in question were formed during a period of 
the earth’s history when the ocean had a considerably different 
relative proportion of mineral substances dissolved in its (then 
probably much warmer) waters ; they are consequently assigned to a 
very early geological period, anterior indeed to what are usually 
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