If, in this short and superficial view of the mineral constituents of the
earth's crust, I do not place immediately after the simple sedimentary rocks
the conglomerates and sandstone formations which have also been deposited as
sedimentary strata from liquids, and which have been imbedded alternately
with schist and limestone, it is only because they contain, together with
the detritus of eruptive and sedimentary rocks, also the detritus of gneiss,
mica slate, and other metamorphic masses. The obscure process of this
metamorphism, and the action if produces, must therefore compose the third
class of the fundamental forms of rock.
Endogenous or erupted rocks (granite, porphyry, and melaphyre) produce, as I
have already frequently remarked, not only cynamical, shaking, upheaving
actions, either vertically or laterally displacing the strata, but they also
occasion changes in their chemical composition as well as in the nature of
their internal structure; new rocks being thus formed, as gneiss, mica
slate, and granular limestone (Carrara and Parian marble). The old silurian
or devonian transition schists, the belemnitic limestone of Tarantaise, and
the dull gray calcareous
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sandstone ('Macigno'), which contains alggae found in the northern
Apennines, often assume a new and more brilliant appearance after their
metamorphosis, which renders it difficult to recognize them.
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