eposition, and thus gives us some indication of the lapse of
geological time from a comparison of this rate with the observed thickness
of stratified rocks on the earth's surface.

{106}

_How to Estimate the Thickness of the Sedimentary Rocks._--The sedimentary
rocks of which the earth's crust is mainly composed consist, according to
Sir Charles Lyell's classification, of fourteen great formations, of which
the most ancient is the Laurentian, and the most recent the Post-Tertiary
or Pleistocene; with thirty important subdivisions, each of which again
consists of a more or less considerable number of distinct beds or strata.
Thus, the Silurian formation is divided into Upper and Lower Silurian, each
characterized by a distinct set of fossil remains, and the Upper Silurian
again consists of a large number of separate beds, such as the Wenlock
Limestone, the Upper Llandovery Sandstone the Lower Llandovery Slates, &c.,
each usually characterised by a difference of mineral composition or
mechanical structure, as well as by some peculiar fossils. These beds and
formations vary greatly in extent, both above and beneath the surface, and
are also of very various thicknesses in different localities. A thick bed
or series of beds often thins out in a given direction, and sometimes
disappears altogether, so that two beds which were respectively above and
beneath it may come into contact. As an example of this thinning out,
American geologists adduce the Palaeozoic formations of the Appalachian
Mountains, which have a total thickness of 42,000 feet, but as they are
traced westward thin out till they become only 4,000 feet in total
thickness. In like manner the Carboniferous grits and shales are 18,000
feet thick in Yorkshire and Lancashire, but they thin out southwards, so
that in Leicestershire they are only 3,000 feet thick; and similar
phenomena occur in all strata and in every part of the world. It must be
observed that this thinning out has nothing to do with denudation (which
acts upon the surface of a country so as to produce great irregularities of
contour), but is a regular attenuation of the layers of rock, due to a
deficiency of sediment in certain directions at the original formation of
the deposit. Owing to this thinning out of stratified rocks, they are on
the whole of far less extent than is usually supposed. When we see a
geological map showing successive formations following each other in long
irregular belts acros