Murchison’s Siluria. 397 
ical and subaqueous origin of the beds is obvious, and from the 
latter to localities where the same strata are wholly unchanged, 
and contain organic remains. ‘Transitions are thus seen from 
ime or gypsum, or shale into mica-schist, as is seen in the sec- 
ondary and tertiary rocks of the Alps.* 
Elementary works will have, indeed, informed the student, 
that such changes of the original sediment have been generally 
accounted for by the influence of great heat proceeding from the 
interior of the earth, and which at different former periods mani- 
fested its power in the eruption of granites, syenites, porphyries, 
amorphism of the original strata has been carried in mountain- 
chains, and at different periods through all formations, though of- 
ten probably connected with such igneous outbursts, must have 
resulted from a far mightier agency than that which was produc- 
five of the mere eruptions of molten matter or igneous rocks. 
The latter are, in fact, but partial excrescences in the vast spread 
of the stratified crystalline rocks,—symptoms only of the grand 
changes which resulted from deep-seated causes; probably from 
the combination of heat, steam, and electricity, acting together 
With an intensity very powerful in former periods 
rocesses now going on in nature on a small scale, or imitated 
poenomena,. j a 
But speculations on such physical operations as those which 
have affected the surface of the earth, are not here called for. At 
all events, the earliest of the phenomena, with which alone we 
are at present concerned, or the first formation of the known crust 
of the planet, belongs to a period in which no definite order, 
= See Alps, Appenines, &c., Quarterly Journal Geol. Soc. Lond,, vol. v, p. 157, 
seq. 
