SCARBA. GEOLOGY. 179 



ceous schist, may be supposed to have separated from 

 those that form the principal parts of quartz rock, which 

 have in water a greater tendency to gravitate; so the 

 finer clay from which clay slate is formed, has probably 

 been separated from the coarser fragments and particles 

 that constitute the numerous varieties of graywacke. 

 Both receive illustration from the association of conglo- 

 merates with finer sandstones in the case of the secondary 

 strata ; the analogy in general being considerable, and the 

 principal differences appearing to consist in changes, pro- 

 duced through long continued periods of repose, or under 

 circumstances with which we are unacquainted . Since also 

 the alternation of conglomerates and sandstones is irre- 

 gular, and that each of these occupies by turns the highest 

 and the lowest relative position, there is no reason to be 

 surprised at the alternations of quartz rock and micaceous 

 schist, or of clay slate and graywacke. 



It must now be remarked, that in the case of the older 

 stratified rocks, the alternations are even more general, 

 since all the four substances just enumerated are in these 

 islands found to alternate without any regular order ; the 

 graywacke and clay slate being irregularly intermixed 

 with the other two rocks, and it being impossible to 

 assign the position of inferiority to any one of the whole 

 set. The evidence for this uncertainty of position, in 

 Scarba, in Isla, and in Jura, is so perfect, and so obviously 

 free from the possibility of mistake, that it cannot be 

 evaded by any supposition. An unbiassed observer would 

 not indeed consider it as a matter either of doubt or dis- 

 cussion, nor is there a priori any ground for doubt, since 

 it does not present any improbability. Where clay, mica, 

 and quartz sand have been gradually deposited, produced 

 as they must apparently have been from the waste of 

 more ancient rocks, and where the particles of these 

 different substances have been reduced to different de- 

 grees of fineness, it is rather more surprising that the 

 alternations and mixtures are not more frequent, than 



