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CHAP IV.

SERIES OF STRATIFIED ROCKS.

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