some of the strata through which these dikes cut are occasionally altered at the
point of contact, as if they had been exposed to the intense heat of melted
matter.

The absence of cones and craters, and long narrow streams of superficial lava,
in England and many other countries, is principally to be attributed to the
eruptions having been submarine, just as a considerable proportion of volcanoes
in our own times burst out beneath the sea. But this question must be enlarged
upon more fully in the chapters on Igneous Rocks, in which it will also be
shown, that as different sedimentary formations, containing each their
characteristic fossils, have been deposited at successive periods, so also
volcanic sand and scoriae have been thrown out, and lavas have flowed over the
land or bed of the sea, at many different epochs, or have been injected into
fissures; so that the igneous as well as the aqueous rocks may be classed as a
chronological series of monuments, throwing light on a succession of events in
the history of the earth.

PLUTONIC ROCKS (GRANITE ETC).

We have now pointed out the existence of two distinct orders of mineral masses,
the aqueous and the volcanic: but if we examine a large portion of a continent,
especially if it contain within it a lofty mountain range, we rarely fail to
discover two other classes of rocks, very distinct from either of those above
alluded to, and which we can neither assimilate to deposits such as are now
accumulated in lakes or seas, nor to those generated by ordinary volcanic
action. The members of both these divisions of rocks agree in being highly
crystalline and destitute of organic remains. The rocks of one division have
been called Plutonic, comprehending all the granites and certain porphyries,
which are nearly allied in some of their characters to volcanic formations. The
members of the other class are stratified and often slaty, and have been called