SUCCESSION AMONG ROCKS. 283 



this partiality will be justified to the reader; while,, as 

 far as regards the secondary strata at least, no ex- 

 amples equally extensive have been brought forward, 

 in which the order of arrangement and succession has 

 been so satisfactorily ascertained. 



In examining this order of succession in the primary 

 class, it is necessary to commence from granite, where- 

 ever that is present; and, when it is not, from the 

 lowest visible rock. Whether this method is, in all 

 cases, unexceptionable or not, we must adopt it, for 

 want of other more certain criteria of the proper 

 points of departure. The following examples, out of 

 many more that might have been adduced, will 

 answer all the purposes at present in view. 



Shetland offers some of the best examples of series 

 of strata in which the members are, at the same time, 

 numerous, and disposed with a considerable degree of 

 that regularity which was once thought universal and 

 necessary. In that part called the Mainland, the gra- 

 nite is followed by gneiss and hornblende schist, quartz 

 rock, chlorite schist, and argillaceous schist ; beds of 

 limestone occurring more than once in the series, both 

 with the gneiss and the micaceous schist. The lowest 

 red sandstone, as the first of the secondary strata, 

 succeeds to the argillaceous schist. In this enume- 

 ration, I have, however, given to the order, the ad- 

 vantage derived from the omission of the minuter 

 alternations ; or treated the principal rocks as so many 

 distinct series, with minor quantities of others in sub- 

 ordination. The enumeration, by beds, of each, in 

 the manner followed by Ebel, and of which I shall 

 give some examples hereafter, would have presented a 

 very different aspect. 



In the same islands, taking Yell, Unst, Fetlar, 

 and the adjoining 1 smaller isles, as one tract, which 



