606 



CLAY, MARLE, AND SAND. 



SOME of these substances are of an alluvial 

 origin, and are therefore treated of in the chapter 

 appropriated to that subject. As others, how- 

 ever, are found interstratified with various rocks, 

 in several parts of the secondary series, they are 

 more properly introduced in a separate place. 

 This is the more necessary, as the two latter, in 

 many cases, are partially solidified so as to form 

 rocks ; offering an exact analogy to that which 

 happens in similar transitions between the clays 

 and shales. They might perhaps with greater 

 propriety have been placed in the secondary class, 

 but, not having universally the general, characters 

 of rocks, it seemed at least equally convenient to 

 include them in this Appendix. 



The clays are found in beds of greater or less 

 dimensions, partially independent, or minutely 

 interstratified with limestone, sandstone, and 



