ENDLicH.] DIVISION OF ROCKS CRYSTALLINE AGGREGATES. 91 



haps, will prove to be so favorable to the study of "form," both massive 

 and in detail, as Colorado. The former, the massive features, are not 

 considered here, but only the result of atmospheric and other erosive 

 inflneuces upon strata or rocks of different physical constitution and 

 different structural conditions. 



All rocks and stratoid deposits can appropriately be divided into four 

 groups, taking their physical characters and their genesis as a basis. 

 [a.) Crystalline aggregates, comprising all those consisting entirely of 

 several crystallized or crystalline minerals, the nature of which 

 determines the species of the rock. 

 (&.) Sedimentary rocks. 



(1.) Deposited without the aid of any chemical separation or 



action whatever. Purely mechanical deposits. 

 (2.) Deposited with the aid of chemical action. Crystalline at 

 times, consisting of but one mineral. 

 (c.) Eruptive rocks, containing segregated minerals in a paste which may 



be amorphous or crystalline. 

 (d.) Eedeposited, recent material. 



This general division with but slight exceptions corresponds in the 

 main to a chronological separation. Instances occur where eruptive 

 rocks, younger than a portion at least of the sedimentaries, consist en- 

 tirely of a crystalline aggregate. Eruptive granite, which is known to 

 have intruded into Post-Cretaceous strata, is an example of this kind. 

 With such and similar exceptions, however, whicli at best occur but 

 rarely, the arrangement is essentially one that corresponds with the suc- 

 cessive genesis of geological divisions. 



CRYSTALLINE AGGREGATES. 



As belonging to this group we count Granites, Syenites, Diorite?, Gab- 

 bro, and Schists. Each one of these is represented by a large number of 

 varieties, determined by the absence or substitution of one or the other 

 constituent mineral. The three lirst of these are, physically, closely 

 related to each other, while the two last form a correspondingly parallel 

 subgroup. Three varieties can be distinguished of the first subgroup, 

 varieties that are based not upon raineralogical or chemical, but purely 

 physical distinctions, coarse, middle, and fine- grained. 



Coarse-grained rocks belonging to this series are by far less able to 

 withstand erosion and degradation than the other two. Frequently 

 either the feldspar or mica may be found to be so arranged as to 

 present its cleavage-plane in one direction mainly. Both of these min- 

 erals show large crystals, and it will be a comparatively easy matter 

 for either flowing water, frost, or any similarly acting agent, to destroy 

 in a short time the original shape of the exposed mass. Should even 

 the cleavage-planes not be so arranged as to fall mainly in one direction, 

 the same causes will produce an abrasion which stands in direct pro- 

 portion to the average size of the crystalline masses, as compared to 

 others. It is evident, therefore, without going into details as regards 

 the process by which the removal of material is effected, that a coarse- 

 grained granite, for instance, will have a tendency to form rounded 

 hills, blunt summits, and rounded edges. In case the crystals lie in one 

 cleavage-plane, frequently small vertical or sloping smooth walls will 

 be formed, which more effectually resist disintegration than the remain- 

 ing portions. One of the most powerful agents in shnping the minor 

 orographic details is the growing vegetation. Through the growth of 

 roots, the enormous wedging-power of which is well known, particles of 



