CHAP IV.
BY following the methods previously described (pages
18, 19.) the whole series of strata existing in any country
can he known; by comparing the results thus obtained
in different countries, the extent of the strata, and the
degree of generality of the causes concerned in producing
them, can also be known. The investigations,
in both respects, have now proceeded so far as to fully
justify a geologist in asserting, that the principal features
of the stratified rocks in the crust of the globe
are very similar over large regions; the aggregate thickness
of their mass is everywhere limited to a few
miles; the order of succession among the principal
groups is the same, or analogous; even the minute
variations of their composition, aggregation, and structure
are observable in remote situations; their organic
contents are reducible to the same schemes of classification,
and everywhere indicate several great physical
changes on the surface of the globe, since it became
the theatre of vegetable and animal life.
The foundations of all sound generalisations in geological science are accurate and mutually explanatory sections and maps of the whole series of stratified and igneous rocks existing in each natural geological district; a term by which we wish to express a part of the earth's crust, whether large or small, in which the formation of aqueous deposits has followed, amidst many local irregularities, one general law of succession. Such sections and maps express, by one common type or formula, the general result of many separate and local investigations; the principal local deviations from the general type must be on no account omitted, for these
