396 Murchison’s Siluria. 
with the difficult problem of ‘the first conditions of our planet. 
Most of them likewise have believed that all the great outbursts 
of igneous matter, by which the crust has been penetrated and 
its surface diversified, were merely outward signs of the contin- 
ued internal activity of that primordial heat, now much repressed 
by the accumulations of ages, and of which our present volcanoes 
are feeble indications. If, then, the mathematician has correctly 
explained the causes of the shape of the globe, the geologist con- 
firms his views when, exatnining into the nature of its oldest 
massive crystalline rocks, he sees in them clear proofs of the 
effects of intense heat. This original crust of the earth was sub- 
sequently, we may believe, broken up by protruded masses, which 
issuing in a melted condition, constituted the axes and centres of 
mountain chains. Each great igneous eruption gave out substan- 
ces that became, on cooling, solid rocks, which, when raised into 
the atmosphere, constituted lands that were exposed to innumer- 
able wasting agencies; and thus afforded materials to be spread 
out as deposits upon the shores and bed of the ocean. In these 
hypothetical views concerning the production of the earliest sed- 
iments formed under water, we seem to reach a primary source ; 
and once admitting that large superficial areas were originally oc- 
cupied by igneous rocks, we have in them a basis from which 
the first sedimentary materials were obtained. 
The earlier eruptions having necessarily occasioned elevations 
at some points and collapses or depressions at others, such chan- 
ges of outline, aided by the grinding action of water, would oc- 
casion the formation of bands of sediment, which, adapting them- 
selves to the inequalities of the surface, must have been of une- 
qual dimensions in different parts of their range. In this way, 
we may imagine how, by a repetition of the processes of eleva- 
tion and denudation, the earliest exterior rugosities of the earth 
would be in same places increased, while in others they would 
be placed beyond the influence of sedimentary accumulation. 
May we not also infer, that the numerous molten rocks of great 
dimensions which were suddenly evolved from the interior at 
subsequent periods, must have made enormous additions to the 
solid crust of the earth, and have constituted grand sources for 
the augmentation of new strata ? 
Turning from the igneous rocks to crystalline stratified depos- 
its, we now know that a great portion of the micaceous schists, 
chloritic and quartzose rocks, clay-slates, and limestones, once 
called primary, were of later origin. Many of these are nothing 
more than subaqueous sediments of various epochs, which have 
been altered and crystallized at periods long subsequent to their 
accumulation. This inference has been deduced from positive 
ob tion. Rocks, for example, have been tracked from 
districts where they are crystalline, to spots where the m 
