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346 Huttonian Theory. ftrudture. To this is fuperadded a flaty ftrudture. Plence the Huttonians infer, that thefe rocks mult be mechanical depofites fomewhat changed by the adtion of heat, becaufe rocks, formed from a ftate of complete fufion, according to their fy- ftem, cannot retain the flaty ftrudture to any great extent. If this inference were corredt, we Ihould never find porphyry-flate, clay-flate, mica-flate, and flaty-quartz, in veins, becaufe, according to the Huttonian Theory, all veins have been filled by the injedtion of fluid matter from below. It is well known, however, that all thefe rocks do occur in veins, which are often of immenfe mag nitude, and iflue from the moft diltindly ftratified rocks. Thefe fadts then demonftrate, that, al though gneifs and mica-flate have a flaty ftrudture, they are not mechanical depofites, and therefore are not compofed of materials older than them- felves; and confequently, that there exifts a clafs of rocks whofe origin cannot be traced to any thing antecedent. This point may be ftill more minutely illuftrat- ed by a comparifon of the ftrufture of gneifs and mica-flate, with fome lock, as Sandftone, which is univerfally admitted to be a mechanical forma tion. Sandftone is compofed of particles of quartz, fometimes alfo particles of felfpar and mica. Thefe particles, when clofely examined, bear all the marks of attrition, and are always connedted together into a mafs, by mean* of a cement which is either of