2 CLASSIFICATION OF ROCKS. [Ca I, 



of circumstances, and at successive periods, during each of which distinct 

 races of living beings have nourished 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, or on which we are 

 enabled to reason by observations made at or near the surface. These 

 reasonings may extend to a depth of several miles, perhaps ten miles ; 

 and even then it may be said, that such a thickness is no more than ^^ 

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

 extent in relation to man, and to the organic beings which people 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. 



The materials of this crust are not thrown together confusedly ; but 

 distinct mineral masses, called rocks, are found to occupy definite spaces, 

 and to exhibit a certain order of arrangement. 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 endeavored 

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

 ponent materials of the earth as consisting of rocks and soils. But there 

 is often so insensible a passage from a soft and incoherent 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 Italian, 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. 



The most natural and convenient mode of classifying the various rocks 

 which compose the earth's crust, is to refer, in the first place, to their 

 origin, and in the second to their relative age. I shall therefore begin 

 by endeavoring briefly to explain to the student how all rocks may be 

 divided into four great classes by reference to their different origin, or, in 

 other words, by reference to the different circumstances and causes by 

 which they have been produced. 



The first two divisions, which will at once be understood as natural, 

 are the aqueous and volcanic, or the products of watery and those of 

 igneous action at or near the surface. 



Aqueous rocks. — The aqueous rocks, sometimes called the sedimentary, 

 or fossiliferous, cover a larger part of the earth's surface than any others. 

 These rocks are stratified, or divided into distinct layers, or strata. The 

 term stratum means simply a bed, or any thing spread out or strewed 

 over a given surface ; and we infer that these strata have been generally 

 spread out by the action of water, from what we daily see taking place 

 near the mouths of rivers, or on the land during temporary inundations. 



