14 



LYELL'S ELEMENTS OF GEOLOGY. 



Successive Formation of the Earth's Crust. 



come to a different conclusion. They have discovered proofs 

 that the external parts of the earth were not all produced in the 

 beginning of things, in the state in which we now behold them, 

 nor in an instant of time. On the contrary, they have acquired 

 their actual configuration and condition gradually, under a great 

 variety of circumstances, and at successive periods, during each 

 of which distinct races of living beings have flourished on the 

 land and in the waters, the remains of these creatures still lying 

 buried in the crust of the earth. 



By the " earth's crust," is meant that small portion of the 

 exterior of our planet which is accessible to human observation. 

 It comprises not merely all of which the structure is laid open 

 in mountain precipices, or in cliffs overhanging the river or the 

 sea, or whatever the -miner may reveal in artificial excavations ; 

 but the whole of that outer covering of the planet on which we 

 are enabled to reason by observations made at or near the sur- 

 face. These reasonings may extend to a depth of several miles, 

 perhaps ten miles ; but even then it may be said, that such a 

 thickness is no more than ^^o^h part of the distance from the 

 surface to the centre. The remark is just ; but although the 

 dimensions of such a crust are, in truth, insignificant when com- 

 pared to the entire globe, yet they are vast and of magnificent 

 extent in relation to man, and to the organic beings which peo- 

 ple our globe. Referring to this standard of magnitude, the 

 geologist may admire the ample limits of his domain, and admit, 

 at the same time, that not only the exterior of the planet, but the 

 entire earth, is but an atom in the midst of the countless worlds 

 surveyed by the astronomer. 



Now the materials of this crust are not thrown together con- 

 fusedly, but distinct mineral masses, called rocks, are found to 

 occupy definite spaces, and to exhibit a certain order of arrange- 

 ment. " The term rock is applied indifferently by geologists to 

 all these substances, whether they be soft or stony, for clay and 

 sand are included in the term, and some have even brought peat 

 under this denomination. Our older' writers endeavoured to 

 avoid offering such violence to our language, by speaking of the 

 component materials of the earth as consisting of rocks and soils. 

 But there is often so insensible a passage from a soft and inco- 

 herent state to that of stone, that geologists of all countries have 

 found it indispensable to have one technical term to include both, 

 and in this sense we find roche applied in French, rocca in Ita- 

 lian, and felsart in German. 



The beginner, however, must constantly bear in mind, that the 

 term rock by no means implies that a mineral mass is in an 

 indurated or stony condition. 



