enabled to reason by observations made at or near the surface. These reasonings
may extend to a depth of several miles, perhaps ten miles; and even then it may
be said, that such a thickness is no more than 1/400 part of the distance from
the surface to the centre. The remark is just: but although the dimensions of
such a crust are, in truth, insignificant when compared to the entire globe, yet
they are vast, and of magnificent extent in relation to man, and to the organic
beings which people our globe. Referring to this standard of magnitude, the
geologist may admire the ample limits of his domain, and admit, at the same
time, that not only the exterior of the planet, but the entire earth, is but an
atom in the midst of the countless worlds surveyed by the astronomer.

The materials of this crust are not thrown together confusedly; but distinct
mineral masses, called rocks, are found to occupy definite spaces, and to
exhibit a certain order of arrangement. The term ROCK is applied indifferently
by geologists to all these substances, whether they be soft or stony, for clay
and sand are included in the term, and some have even brought peat under this
denomination. Our old writers endeavoured to avoid offering such violence to our
language, by speaking of the component materials of the earth as consisting of
rocks and SOILS. But there is often so insensible a passage from a soft and
incoherent state to that of stone, that geologists of all countries have found
it indispensable to have one technical term to include both, and in this sense
we find ROCHE applied in French, ROCCA in Italian, and FELSART in German. The
beginner, however, must constantly bear in mind that the term rock by no means
implies that a mineral mass is in an indurated or stony condition.

The most natural and convenient mode of classifying the various rocks which
compose the earth's crust, is to refer, in the first place, to their origin, and
in the second to their relative age. I shall therefore begin by endeavouring
briefly to explain to the student how all rocks may be divided into four great
classes by reference to their different origin, or, in other words, by reference