180 UUARTZ ROCK. 



secondary sandstones : which are even found highly 

 crystalline and compact, as in Fife, already noticed, 

 where a bed in the coal strata is a compact quartz 

 without any marks of mechanical origin. The fun- 

 damental difficulty, in all these cases, is that which 

 affects the whole history of silica. It 4ms been once 

 extensively dissolved in water, as is proved by a thou- 

 sand facts ; and is still soluble, if in a less degree, as 

 we know from events of recent or daily occurrence. 

 Such causes may have consolidated many primary 

 deposits of sand into compact quartz rock, whether 

 felspar and mica were ingredients or not. But, as in 

 gneiss and micaceous schist, heat cannot have been 

 inactive, especially where it alternates with these : 

 while it may as well have produced this effect from 

 some general cause, as it has from local causes when 

 trap is present. The quartz bed of Fife is, evidently, 

 a secondary sandstone consolidated by heat : from its 

 position, from the analogy of the cherts thus pro- 

 duced, and, still more, from the fact that, in Sky, the 

 recent sandstones are converted into solid quartz in 

 several places, precisely where they come into contact 

 with veins or masses of trap. In this case, the action 

 of heat is admitted, and the effect in question has 

 taken place ; and, similarly, in Ben-na-chie and in 

 Glen Tilt, where beds of quartz rock are intermixed 

 with granite, they are indurated to compact quartz 

 when pure, and converted into jasper or chert when 

 they have contained felspar or clay ; these changes 

 being all co-extensive with the vicinity of the granite. 

 Here, there are similar effects from a cause which, if 

 not universally admitted to be the action of heat, will 

 not probably for ever be denied. 



The indestructible nature of most of the varieties 

 of this rock, points it out as adapted for architectural 

 purposes, though hitherto neglected. It can also fre- 



