176 tLUARTZ ROCK. 



Had these been found in a secondary sandstone, they 

 would have heen considered as originating in the steins 

 of vegetables: hut until we find more decided organic 

 fragments among the primary rocks, no geologist will 

 receive them as such ; particularly when it is prede- 

 termined that this must not he. Yet if the coal in 

 those rocks has originated in vegetables, these con- 

 cretions may also have been derived from the same 

 source. When it contains cavities lined with crystals 

 of quartz, these must be attributed, as in the trap 

 rocks, to watery infiltrations of silica. Contortions, 

 so common in the primary strata, scarcely ever occur 

 in quartz, rock; a few trifling examples only, having 

 fallen under my notice: but flexures on the large 

 scale are very conspicuous in the island of Scarba. 

 The probable reasons were formerly stated. 



Of all the primary strata, quartz rock presents the 

 greatest regularity of stratification. The evenness, 

 persistence, repetition, and distinctness of the strata, 

 are such, that it is matter of surprise how its true 

 nature could have been mistaken, or how it could ever 

 have been confounded with granite, as it was by the 

 well-known describers of Jura; even had its mineral 

 characters not been so explicit. It is a lesson to 

 those who set about to construct systems of geology 

 from books; but a lesson thrown away: still more 

 thrown away on those who describe what they have 

 not knowledge to see. In this and the associated 

 islands, the Geological student may examine a series 

 of stratification on a gigantic scale, as perfect as if it 

 was displayed in a model; and, for the particulars, I 

 may refer to the account of the Western Islands. 



The thickness of the strata varies, but is more often 



nconsiderable than otherwise; those of a few inches 



or feet in depth being common; as those of manv 



