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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 has 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 he the action of heat, will not probably for ever he 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-