CHERT. 287 



from the point of junction. In these latter examples 

 also, it is frequently impossible to determine whether 

 the rock ought to be referred to jasper or chert ; a 

 condition of things easily understood by recollecting 

 the very variable composition of the later secondary 

 limestones, and the overwhelming proportions of ar- 

 gillaceous and siliceous earth which they often con- 

 tain. If it is superfluous to note the support which 

 this rock also affords to the igneous theory of trap 

 and granite, it is interesting to remark that the very 

 same substance is found in the vicinity of volcanic 

 rocks, under similar circumstances ; as are siliceous 

 schists and jaspers, wherever the requisite original 

 rocks have been present. These have been a great 

 source of trouble to geologists and collectors, by whom 

 they have been ranked with lavas ; a term too often 

 adopted to save the trouble of investigation. That 

 confusion, I have, I trust, now removed, among many 

 more. 



