lime 0.92 ... ... 0.10
Organic matter 0.20 ... ... ...
Water 0.50 ... 0.69 ...
---- ---- ---- ----
99.86 99.74 99.69 99.63

These limestones are hard and possess to a greater or less extent a
crystalline texture. They are replaced in later geological periods by
others which are much softer, and often purer, of which the oolitic
limestones, so called from their resemblance to the roe of a fish, and
chalk are the most important. Other limestones are also known which
contain an admixture of clay. The soils produced by the disintegration
of limestone and chalk are generally light and porous, but when mixed
with clay, possess a very high degree of fertility, and this is
particularly the case with chalk, which yields some of the most valuable
of all soils. But it is true only of the common limestones, for
experience has shown that those which contain magnesia in large quantity
are often prejudicial to vegetation, and sometimes yield barren or
inferior soils.

Such are the general characters of the three great classes of stratified
rocks; any attempt to particularise the numerous varieties of each would
lead us far beyond the limits of the present work. It is necessary,
however, to remark, that in many instances one variety passes into the
other, or, more correctly speaking, sedimentary rocks occur, which are
mixtures of two or more of the three great classes. In fact, the name
given to each really expresses only the preponderating ingredient, and
many sandstones contain much clay, shales and clay slates abound in
lime, and limestones in sand or clay, so that it may sometimes be a
matter of some difficulty to decide to which class they belong. Such
mixtures usually produce better soils than either of their constituents
separately, and accordingly, in those geological formations in which
they occur, the soils are generally of excellent quality. The same
effect is produced where numerous thin beds of members of the different
classes are interstratified, the disintegrated portions being gradually
intermixed, and valuable soils formed.

The fertility of the soils formed from the stratified rocks is also
increased by the presence of organic remains which afford a supply of
phosphoric acid, and which are sometimes so abundant as t