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Ch. XII.] NATURE OF STRATIFICATION. 231 has made unprecedented progress, yet the words stratum, stratified, and stratification are still in use, and have not, even at the present day, been satisfactorily defined. Thus De la Beche *, in his explanation of terms employed in geology, says, “ Although the word stratum should only be applied to a bed of rock, the upper and under surfaces of which are parallel planes; it is also employed to designate beds, the upper and under surfaces of which are irregular. Hence rocks are called stratified, even when the planes of the beds are not precisely parallel to each other.” So, Lyell f remarks, “ When several rocks lie like the leaves of a book, one upon another, each individual forms a stratum.” Need it be observed that the term stratum, according to either of these explanations, is also applicable to the layers or parallel masses of the so-called igneous rocks; which implies a contradiction, for such rocks are generally said to be un- stratified. Macculloch ^ has attempted to meet this difficulty, by extending its signification. “ The term stratum or bed,” lie says, “ carries its own definition with it; its extent, according to the prolongation of its great opposing planes, being gene rally far greater than its thickness: it is distinguished from masses of a similar shape, which occur among the unstratified rocks, by the nature of its origin ; for the word stratification implies a cause, as well as a mode of form or disposition, and that cause is assumed, or proved, to consist in a deposition from water.” It is questionable, however, whether even this addition fairly meets the difficulty of the case: a definition ought to convey such an intelligible description of its subject, as to be easily recognised; but who, not even excepting the most accomplished geologist, can in all cases pronounce whether a * Geological Manual, 8vo. p. 598. f Principles of Geology, vol. iii. p. 81. \ System of Geology, vOl. i. p. 67. Q 4