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. 



MaccullochJ has attempted to meet this difficulty, by 

 extending its signification. " The term stratum or bed," he 

 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 



