ON THE APPARENT DISLOCATIONS [Ch. XV, 



mentary and derivative rocks : such a position would be 

 absurd, and contrary to positive evidence. But it is proposed 

 to enquire, whether the resemblance to fragments and peb- 

 bles observed in many primary, and indeed in some fossili- 

 ferous rocks, may not be only a sportive imitation of nature, 

 a coincidence more apparent than real, and capable of being 

 referred to a cause distinct from that by which the materials 

 of the mechanical rocks were produced. 



Let us begin with the more simple case of the conglomerated 

 appearance: and it is necessary that the general reader 

 should reconsider the details already given on the structure 

 of rocks. 



In the preceding chapters we have endeavoured to show, 

 that the crystalline strata are not sedimentary deposits, sub- 

 sequently altered by Plutonic agency ; but the occurrence of 

 these conglomerates, interstratified with such strata, may ap- 

 pear to be opposed to this conclusion ; at least it is viewed in 

 this light by the most eminent geologists of the day. We, 

 however, appeal from these authorities to nature ; and main- 

 tain, that the nodules of these conglomerates, generally, if 

 not always, possess characters very different from those which 

 occur in true detrital deposits. The latter often consist of 

 various substances, some of one rock and some of another ; 

 exhibiting different forms, according to the nature of the 

 fragments from which they have been formed ; and all these 

 occur in a basis which has seldom the same composition as 

 the included nodules. The former, however, are of a similar 

 nature in the same rock ; and in their constitution partake 

 of the elements not only of the adjacent strata, but also of 

 the basis in which they are imbedded ; and very frequently 

 their component parts are arranged around compact or cry- 

 stalline nuclei ; sometimes, indeed, so regularly, as to assume 

 a perfect concentric structure. Thus, in the case of gneiss, 

 it often happens that the nodules and the basis can only be 

 distinguished from each other by the globular structure of 

 the former ; or, if they differ in composition, as in the Isle of 



