191 



In the stratified rocks, as the sandstones, both 

 red, and white, are found alternating with the 

 shales, so they pass into them by gradations 

 similar to those which occur between the primary 

 sandstone and the argillaceous schist which so 

 often accompanies that rock. 



In the same way, the beds of secondary lime- 

 stone that are interstratified with shales, some- 

 times become so overcharged with argillaceous 

 and siliceous earths, as to produce impure com- 

 pounds scarcely differing from the accompanying 

 shales that are in a similar manner intermixed 

 with calcareous matter. 



The transitions which are to be observed 

 among the multifarious unstratified rocks of this 

 class, do not require notice here : they must be 

 considered as variations of structure and compo- 

 sition in a single family, throughout which a 

 complete system of gradation exists in every part. 



Those associations and transitions which are 

 often very remarkable among sqme of the most 

 important and extensive varieties of the secon- 

 dary strata, are not subjects for notice in this 

 place. They belong to geological systems, or to 



