TERTIARY FORMATION. 



Ill 



It may assist the memory to notice, that the mi- 

 ocene contains a minor proportion, and jp/iocene a 

 comparative ^Zurality of recent species ; and that 

 the greater number of recent species always implies 

 the more modern origin of the strata. At present 

 there are about 9000 species of recent shells dis- 

 covered. 



The application of this principle of classification 

 to the tertiary formation of the United States is 

 now in a course of successful prosecution by Mr. 

 Conrad, Drs. Morton, Harlan, Dekay, Hildreth, and 

 other able paleontologists : so that, at no distant 

 period, we shall doubtless be able to pronounce 

 with as much certainty in respect to the compara- 

 tive ages of our different strata, as they now do in 

 France or England. 



The lower tertiary formation is subdivided into the 

 London clay and the plastic clay; the latter rests 

 upon the chalk, and it is called plastic because, in 

 France, it is extensively used in pottery, though in 

 the environs of London it is composed of beds of 

 flint and pebbles alternating with sands and clay. 

 This deposite is remarkable for the vegetable fos- 

 sils and beds of lignite which it contains. In some 

 parts of England it contains beds of imperfect 

 wood-coal ; also remains of marine animals, though 

 fresh- water shells are intermixed. 



The London clay is placed over the plastic clay 

 and sand ; and is the great, dark-coloured argilla- 

 ceous mass upon which the city of London stands. 

 Its beds are often considerably indurated, and of a 

 slaty structure, and vary in thickness from 100 to 

 400 feet, or more. Although distinct names have * 

 been given to these two clays, yet it is believed 

 that there is nothing in their mineral character 

 which warrants this distinction being kept up, or 

 their being considered other than as a series of 

 clays, where the fossils preponderate in the upper 

 part, and the sand and pebbles in the lower. This 



