670 CHERT. 



I have been under the necessity of introduc- 

 ing into this chapter, as a sort of appendix, a rock 

 for which no other part of the arrangement offers 

 a place ; although it differs in many respects from 

 the substances contained in the preceding cata- 

 logue. As also it has been called a chert by the 

 authors who have described it, the student will 

 naturally turn to this division for it. It is the 

 cavernous siliceous rock of the fresh-water de- 

 posit of Paris, to which the name of Burr-stone 

 is popularly applied. If these deposits shall 

 hereafter call for a separate place in a geological 

 classification, it will be easy to amend this de- 

 fective part of the present arrangement. 



