

LEWIS. GEOLOGY. 



two portions of gneiss, seems proved from its being 

 accompanied by a vein of quartz passing in the same 

 undisturbed direction both through it and the troubled 

 mass of rock in which it lies. It is composed of slightly 

 rounded or angular pebbles of quartz and gneiss im- 

 bedded in a gravelly clay. The neighbourhood no where 

 produces any similar rock, the large tract of conglomerate 

 already described lying on the opposite side of the island. 

 The nature of this substance is obscure. It is not likely 

 to be either a solitary spot of a secondary formation depo- 

 sited as it now lies, or the remains of a larger mass of 

 such strata ; and it can scarcely, from its regular position 

 compared with the irregularity of the surrounding gneiss, 

 be a portion of those beds, or a real alternation. It may 

 possibly have originated in a fissure which has been 

 filled by an accumulation of rolled matter, and subse- 

 quently laid bare on one side by the falling of the 

 boundary. In this case it will afford an example of a circum- 

 stance of considerable rarity, a conglomerated vein. The 

 quartz vein which crosses it may be of more recent origin. 



It is almost superfluous to say that trap veins occur 

 in this as in all the other islands. They are not however 

 numerous, nor did I observe any sufficiently remarkable 

 to deserve particular description. 



Sulphate of Barytes is found in the conglomerate near 

 the Chicken Head, but not in great quantity ; nor does 

 it present any regular crystallizations. 



These remarks terminate the description of the Long 

 Isle ; a tract rendered wearisome by the incessant recur- 

 rence of the same rock and the almost total want of 

 variety which its survey affords. The paths of this 

 science, like the subjects 'of which it treats, are not 

 fertile in flowers, and the gneiss of these islands, exposing 

 bare rocks without marks of vegetation or varied only 

 by the brown moor and quaking bog, is an emblem 

 not unapt of the sterility with which the pursuit too 

 often repays the labour and the patience of the geologist. 



