2 
SILURIA. 
[Chap. I. 
earth was, we may believe, marked by intruded masses issuing, at least 
partly, in a melted condition, and often constituting the axes and centres of 
former mountain-chains very different from those which now exist. Each 
great igneous eruption gave out substances which became, on cooling, solid 
rocks ; and, when raised into the atmosphere with the associated meta- 
morphosed strata, they constituted lands afterwards exposed to innumerable 
wasting agencies, thus affording materials to be spread out as fresh de- 
posits upon the shores and bed of the ocean. In these hypothetical views 
concerning the production of ancient sediments formed under water, we 
seem to reach a primary source, and, once admitting that large superficial 
areas were originally occupied by igneous rocks, we have in them a 
basis from which the first sedimentary materials may have been obtained. 
The earlier eruptions accompanied elevations at some points and col- 
lapses or depressions at others ; these changes of outline, aided by the 
grinding action of waves, would occasion the formation of bands of 
sediment, which, adapting themselves to the inequalities of the surface, 
must have been of unequal dimensions in different parts of their range. 
In this way we may imagine how, by a repetition of the processes of ele- 
vation and denudation, the earliest exterior rugosities of the earth would 
be in some places increased, while in others they would be placed beyond 
the influence of sedimentary accumulation. We may also infer that the 
numerous molten masses of great dimensions which were evolved from the 
interior at subsequent periods must have made enormous additions to the 
earliest formed external crust of the earth, and have constituted grand 
sources for the augmentation of new deposits. 
Turning from the igneous rocks to stratified deposits, we now know that 
vast masses of gneiss, micaceous schists, chloritic and quartzose rocks, clay- 
slates, and limestones, once called ' primary,' are really of subaqueous origin. 
Many of these, indeed, are nothing more than sediments of various epochs, 
which have been altered and crystallized long subsequent to their accumu- 
lation. This inference has been deduced from positive observation. A 
rock, for example, has been tracked from the districts where it is crystal- 
line, to other spots where the mechanical and subaqueous origin of the 
beds is obvious, and from the latter to localities where the same stratum is 
wholly unchanged and contains organic remains. Transitions are thus 
seen from compact quartz-rock, in which the grains of silica are scarcely 
discoverable even with a lens, to strata in which the sandy, gritty, and 
pebbly particles bespeak clearly the original accumulation of the mass 
under water. Equivalent passages occur from crystalline, chloritic, and 
micaceous schists into those clay- slates which are little more than con- 
solidated mud, and from crystalline marble to common earthy limestone, 
in which organic remains abound. This land of metamorphosis compre- 
hends such changes, for example, as those by which ordinary limestone 
has been converted into dolomite and sulphate of lime or gypsum, — also 
