quently be procured in parallel beds of various thick ness ; while, often possessing natural joints, like many other schistose rocks, it breaks readily into fragments, easily trimmed by the hammer. The difficulty of producing a smooth surface by the pick, might limit its uses to certain classes of masonry ; but in those, the facility of giving it a form lit for building, would render it an oeconomical substitute for granite, which it also far excels in durability. As a lining for fur naces, its value is obvious. The want of foreign observations on this rock, pre vents us from knowing whether it is a repository of metals. If it be excluded from the metalliferous rocks because it contains no metals in Scotland, we might draw the same conclusion with regard to many others. Yet at Tyndrum, as noticed under micaceous schist, ga lena and blende are found in a hill which consists of al ternate beds of quartz rock and micaceous schist, and in which the former is nearly as abundant as thclatter. Red primary Sandstone. I have associated this in the same chapter with quartz rock, for reasons which will ultimately appear; but before proceeding to its history, I must limit its range as formerly given in the account of the Western Isles. A re-examination of some of these districts has. shown, more clearly, what I then suspected; that some of the examples then quoted belong to the old red sandstone ; the difficulty of distinguishing them where the secondary follows the primary one, espe cially in conformable order, being extreme, or almost insurmountable. The varieties of this sandstone arc very limited ; nor does it require any further general definition than that of the secondary red sandstone. It is, fundamen tally, an aggregate of quartz and red felspar, varying