LOWEST, OR OLD RED, SANDSTONE. 221 



of opportunity; since the alternations of this rock with 

 gneiss and micaceous schist render that a matter of 

 course, should it occur in such a place. And if also I 

 have seen no instances of its junction with primary 

 limestone, the rarity of this rock equally explains an 

 exception which can be only apparent. 



The strata which succeed to the red sandstone must 

 needs he various; from the different successions of 

 those in different countries, from their variations in 

 number and quality, and from that extenuation through 

 which, even in a considerable series, the upper some- 

 times repose on the boundaries of the containing ca- 

 vity. Yet as the " mountain limestone" is conspicu- 

 ously the next stratum in England, while sufficiently 

 constant in Scotland, and as an analogous one is simi- 

 larly found on the continent, this must be esteemed 

 the natural succession. Yet that any superior beds 

 may follow it, to the exclusion of this limestone, and 

 more, is evidenced in Scotland,where,in different places, 

 it is succeeded by the coal strata, by the red marl, by 

 the lias and oolithe > and by the green sand deposit. 



As this sandstone is composed of the fragments of 

 previous rocks, every antecedent substance is found to 

 aid in its formation; including, with granite and the 

 primary strata, the trap rocks of a more antient date : 

 while I need only add that as the conglomerate and 

 coarser beds contain fragtnents of rocks, and the more 

 conspicuous minerals of these, quartz and felspar, so 

 the finer are the produce of sand or of sand and clay, 

 with, more rarely, rnica and calcareous matter. All 

 these substances however are not every where con- 

 tained in this rock; which, on the contrary, in different 

 places, presents great diversities of composition. These 

 variations are matters of interest, independently of the 

 merely mineral differences ; since they are connected 



