52 M. Mobs' Summary of Geoynostical Phenomena. 



sequence of particular circumstances. However, this is not 

 universally the case. But in cases where it is, such places, viz. 

 at the edges of the basin, are the best suited for the investiga- 

 tion of these rocks, and for determining their succession in the 

 basin. At the same time, the eminences and the hollows 

 constituting the valleys are not to be neglected, although both 

 in such tracts are generally less important in throwing light 

 on the subject, than in actual mountainous districts. 



Case in which Fossil Organic Remains may be used for 

 assisting in the determination of formations. — It is quite a na- 

 tural consequence of the constitution of the so-termed se- 

 condary series, and of their mode of superposition, that the 

 thread of connection is easily lost, by means of which we 

 can hope to attain a true knowledge and correct apprehen- 

 sion respecting them ; for, in respect to their rocks, they con- 

 tain but little variety, as they chiefly consist of limestones, in- 

 cluding delomite, and of sandstones ; and they frequently oc- 

 cupy a horizontal position, and one of their rocky masses covers 

 the others for so great an extent that we can see nothing of 

 them, and can ascertain nothing respecting them by direct 

 observation, and are even unable to form a correct opinion of 

 the overlying rock, because, very often, this can only be done 

 by means of the underlying strata. It is in such circumstances 

 that, among other auxiliaries, which cannot often be applied 

 without some hesitation, petrifactions may be of the ut- 

 most service ; and it is hence indispensably necessary to ob- 

 serve them with all care. But this is the only use which 

 geognosy can or ought to make of these remains of organic 

 beings for the determination, or in fact for the recognition, 

 in one and the same basin, of a rocky mass which had been 

 already determined; although these remains are likewise of 

 great importance in other scientific respects, which cannot 

 here be taken into consideration. For example, if we ob- 

 serve a certain mass of limestone in its succession below, 

 between, or above other rocky masses, and properly deter- 

 mine its relations of superposition ; and if, in the same lime- 

 stone, we find certain petrifactions which are peculiar to it, 

 and are generally termed characteristic ; and if, in the same 

 basin, we find a limestone, whose underlying and overlying 



