200 SOIL. 



CHAPTER v. 



The solid mass of our earth does not everywhere present the 

 same physical characters, or the same chemical composition. Tn 

 traversing a mountainous country of any extent, we seldom fail to 

 observe a notable difference in the nature and relative position of the 

 rocks which compose it; the idea which forces itself upon the mind 

 in such circumstances is, that these mineral masses have not had 

 the same origin, that they have been formed and placed in their 

 several situations at distinct and often distant epochs. 



In examining attentively the inequalities which mark the surface 

 of the globe, we soon perceive that those rocks which generally 

 form the most elevated points, the axis or skeleton of mountain 

 chains, result from the agglomeration or intimate mixture of different 

 mineral substances which may be isolated and separately studied. 



These crystalline masses are frequently covered to a certain 

 depth, and even completely concealed by rocks of more recent for- 

 mation, the fragmentary elements of which proclaim their origin 

 from the attrition or breaking down of the strata wliich support 

 them. The regular stratification of these superimposed rocks, the 

 configuration of their minute particles, the remains of organized beings 

 which are found in them, proclaim them to be deposites which have 

 taken j)lace successively, and from the ocean. Ti»e formation of the 

 crystalline rocks probably dates from the period at which the crust 

 of the globe became solid. These elements, intimately mingled by 

 fusion, combined as they cooled, according to the laws of allinity, to 

 constitute the mineral species which we encounter; just as it hap- 

 pens that mineral species, identical with those which we observe in 

 nature, are j)ro(luced and crvstallize during the consolidation of cer- 

 tain scoriae from our furnaces. 



The various circumstances which have accompanied the cooling 

 o*" the crust of the globe, have doubtless occasioned the differences 

 wnich we observe in the distribution of the minerals that enter into 

 the composition of rocks. Thus granite and mica schist, which pre- 

 sent so dissimilar a structure, are nevertheless, and very certamly, 

 varieties of the same species, and contain quartz, felspar, and mica. 

 In sienile,the mica is replaced by amphibolite, and in j/rotogenite by 

 talc. In trachite, a volcanic rock, both of t)M«T and n)t»re recent 

 date, quartz is almost entirely wanting ; the amjilnboliie is replaced 

 by pyroxenite, and the felspar which is er)counlPii;d, is no longer 

 identical in its chemical composition witli that wbirli enters into the 

 constitution of granite. The liiucstttne rock, wbivh l.elonijs to the 

 same Plutonic epoch, is granular or saccharoid ; occasionally ihe 

 intervention of magnesia makes it pass into Jolornite. 



The sedimentarv strata do n«»i varv less ir ih^ir composition Tb« 



