GEOLOGY OF BUTE. 



323 



chalk to the state of a saccharine marble, spreads out beyond 

 the limits of the chalk, and overlies and alters the new red 

 sandstone and coal measures, just as traps rising through the 

 Old Red in Clydesdale overflow upon the coal formation. 

 Such overlying masses must also belong to the tertiary 

 period: and the distribution of this basaltic series over the 

 western isles, in direct continuation of those of north-east 

 Ireland, long ago led to the conclusion that they were of the 

 same age with them, and that this vast area, ranging from 

 Skye to Londonderry, had been once the scene of contempor- 

 aneous volcanic activity. A step in the wrong direction, 

 was taken by Edward Forbes, when mistaking the structure 

 of Trotternish, he endeavoured to shew that the traps there 

 were of oolitic age. This mistake was several years ago 

 pointed out by us, and the deceptive appearances explained. 

 It is now, and we think rightly, agreed, mainly from the 

 evidence of the lignites and leaf-beds, that the entire series 

 belongs to the middle tertiary that is, miocene period. 

 Conybeare and Buckland, half a century ago, referred the 

 traps of north-east Ireland and the Scottish Isles to one 

 era of eruption, of later date than the chalk ; and it is not 

 improbable that these Bute lignites, ochres, and traps, though 

 overlying carboniferous beds, may really be of the same tertiary 

 age. The wood is so much altered that we have failed to 

 determine its true character; but as Professors Harkness 

 and Blyth, by microscopic and chemical examination, have 

 shewn the identity of the Mull and Antrim lignites (Ed. 

 Phil. Jour. N.S., vol. iv. p. 304, 1856), there can be little doubt 

 that those of Bute are the same and so presumably of the 

 same age. 



136. The chemical composition of ochre is almost exactly 

 the same as that of basalt: 100 parts consists of silex, 5640 ; 

 alumina, 3'46 ; per-ox. iron, 24-14; carb. lime, 0'90 ; water, 

 15 -10. It is, in fact, a decomposed basalt, or volcanic ash, 

 pai-tially re-fused and reconsolidated by the succeeding flow 

 of igneous matter. 



