schists commonly present the general aspect of the argillaceous ones, and possess a much greater degree of induration, which, at the same time, does not arise from the predominance of quartz. It is necessary to avoid confounding them with the hard argillaceous schists which derive that property from this mineral, and which have often been mistaken for real siliceous schist by geologists of reputation. It is now almost superfluous to observe that they often contain horn blende and thus pass into hornblende schist. Those of the secondary class are far more distinguished from the shales which they accompany, and approximate more nearly to the igneous rocks to which they owe their origin. But, even in these, the schistose, or the stratified disposition, can generally he discovered in nature, though it may not he apparent in a hand spe cimen. If other substances alternate with them, if they present laminae of different colours, or contain organic remains, they are easily distinguished from the trap rocks, even when detached from their con nexions. It is scarcely possible to confound them with any other rocks ; from the whole of which they are distinguished by their great brittleness, the clean ness and sharpness of their fracture, and the absolute uniformity of their texture ; the surfaces being fre quently as fine and smooth as those of common flint. That they sometimes contain the organic remains of the original shales, though much obscured, was al ready noticed ; and thus have incautious observers described shells as found in basalt. Chert. If the very limited extent of this rock may seem, to the superficial observer, to render it insignificant, the peculiar circumstances under which it occurs give it a