TERTIARY FORMATION. Ill 
It may assist the memory to notice, that the mi- 
ocene contains a minor proportion, and pliocene a 
comparative ;?/urality 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 ia 
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 
clays, where the fossils preponderate in the upper 
part, and the sand and pebbles in the lower. This 
