EXCURSION XV. 189 



We often find that, either from their successive erosion, 

 or from some inequality of original deposition, these beds 

 thin off at the out-crop, just as rocky strata do; and thus 

 the beds are brought so close together that the distinction is 

 not perceived by an unpractised observer; and that shells 

 from an upper bed get on to the surface of a lower, or 

 perhaps into its very substance. This source of error is 

 most likely to be met with in the case of basin-shaped or 

 nearly level deposits. We think it most important that 

 all who engage in the study of these most perplexing 

 superficial formations should guard against this source of 

 error and those already indicated (Art. 85) ; they ought 

 first to study the mineral character, and then the order 

 of the beds, and not hastily assign to the boulder-clay that 

 is, the lowest and oldest bed shells or other fossils which 

 may really belong to those which are superior to it. We 

 have never found fossils in this oldest bed, while we were the 

 first to shew that marine shells and elephantine remains 

 occur under it. We by no means assert that fossils may not 

 exist in it, yet we cannot help thinking that far greater 

 caution should be used in assigning such to this bed than 

 has hitherto been shewn. Ita upper surface undulates, and 

 the shell bearing hollows of the bed above might readily be 

 taken as belonging to this.* We cannot theorize with any 

 certainty respecting the origin of these deposits until, by 

 a more careful study and extended examination, we get 

 hold of a number of great general facts in regard to the 

 physical order of the beds, their internal structure, and 

 fossil contents. In regard to the fauna of the Arran beds, it 

 will have been observed that it is all of one character ; the 

 same species occur in all the beds, with slight local excep- 

 tions. The shell-bed itself is of uniform character, with 

 very slight variations; and hence it follows that uniform 



* Let every student before entering on these beds peruse carefully a 

 most lucid and instructive paper by the Rev. H. W. Crosskey, on 

 their structure and relations (Tr. Phil. Soc., Glasgow Sess. 1868-69). 



