alogous to that
observed in the geographical distribution of species now living.

CHAPTER XI.

ON THE SUPPOSED FORMER INTENSITY OF THE IGNEOUS FORCES.

Volcanic action at successive geological periods--Plutonic rocks of
different ages--Gradual development of subterranean
movements--Faults--Doctrine of the sudden upheaval of parallel
mountain-chains--Objections to the proof of the suddenness of the
upheaval, and the contemporaneousness of parallel chains--Trains of
active volcanoes not parallel--As large tracts of land are rising or
sinking slowly, so narrow zones of land may be pushed up gradually
to great heights--Bending of strata by lateral pressure--Adequacy of
the volcanic power to effect this without paroxysmal convulsions.

When reasoning on the intensity of volcanic action at former periods, as
well as on the power of moving water, already treated of, geologists
have been ever prone to represent Nature as having been prodigal of
violence and parsimonious of time. Now, although it is less easy to
determine the relative ages of the volcanic than of the fossiliferous
formations, it is undeniable that igneous rocks have been produced at
all geological periods, or as often as we find distinct deposits marked
by peculiar animal and vegetable remains. It can be shown that rocks
commonly called trappean have been injected into fissures, and ejected
at the surface, both before and during the deposition of the
Carboniferous series, and at the time when the Magnesian Limestone, and
when the Upper New Red Sandstone were formed, or when the Lias, Oolite,
Green Sand, Chalk, and the several tertiary groups newer than the chalk,
originated in succession. Nor is this all: distinct volcanic products
may be referred to the subordinate divisions of each period, such as the
Carboniferous, as in the county of Fife, in Scotland, where certain
masses of contemporaneous trap are associated with the Lower, others
with the Upper Coal measures. And if one of these masses is more
minutely examined, we find it to consist of the products of a great many
successive outbursts, by which scoriae and lava were again and again
emitted, and afterwards consolidated, then fissured, and finally
traversed by melted matter, constituting what are called dikes.[239] As
we enlarge, therefore, our knowledge of the ancient rocks formed by
subterranean heat, we find ourselves compelled to regard them as the
aggre