CHERT. 283 



schists commonly present the general aspect of the 

 argillaceous ones, and possess a much greater degree 

 of induration, which, at tlie 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 be discovered in 

 nature, though it may not be 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 



