286 CHERT. 



in this case, we have no means of judging. The chert 

 of the secondary limestones may as readily be traced 

 into these as the siliceous schists may into the shales : 

 and they vary in aspect and character, as the lime- 

 stones from which they are derived have contained a 

 greater or less proportion of foreign ingredients. In 

 many cases, the same limited specimen presents an 

 alternation of the siliceous schist and the chert toge- 

 ther ; the adhesion being produced by the causes to 

 which both the substances owe their induration. I 

 know not that organic remains have ever been found 

 in these cherts, although they exist in the associated 

 shales. From the phenomena, indeed, occurring in 

 all cases where beds of limestone lose their form and 

 original character, through their vicinity to igneous 

 rocks, those remains appear always to vanish in these 

 instances ; or, at least, their forms are so changed as 

 to be no longer recognisable ; all the materials of the 

 rock having probably entered into one new combina- 

 tion. Secondary chert necessarily occurs in beds, as 

 the limestone whence it is derived does. Within my 

 experience, these are not very thick ; nor is that to be 

 expected ; since the influence of heat cannot be sup- 

 posed capable of extending deeply through a bad con- 

 ducting medium ; and since, in the accompanying sili- 

 ceous schist, the change is often limited to a certain 

 portion of the bed in contact with the trap. 



A species of chert is sometimes also produced, as I 

 have already noticed, by the same influence exerted 

 on calcareous sandstones. The character of these is 

 rarely so perfect ; and, not unfrequently, they can 

 only be considered as varieties which have acquired 

 an unusual degree of induration. Like the former, 

 they occur near veins of trap ; but the changes, as 

 might be expected, are limited to a short distance 



