ORGANIC REMAINS. 101 
remains are different in the different strata. In the transition 
rocks are found various kinds of coral, madrepore, crinoidae, tri- 
lobites, and other similar organic substances, very many of which 
are altogether different from such as are known to exist in the 
ocean at the present day. In many cases, not only has the spe- 
cies or genus, but the type or general structure and form disap- 
peared, so that amongst the living races there is none that bears 
more than a distant resemblance to such as filled in countless mul- 
titudes the waters of that ancient earth. The old red sandstone 
and metalliferous limestone which rest upon the transition rocks, 
also contain these remains, and in greater abundance, but they 
are of a different kind, bearing however, a greater resemblance 
to such as are found in the transition strata than to the tribes now 
inhabiting the sea. The coal measures abound in fossil remains 
and impressions of plants, but shells are rare, and in many cases 
altogether wanting. The. Magnesian limestone which comes 
next has many shells, petrified fishes, and the remains of an am- 
phibious animal of the genus Monitor are found in it. In the 
new red sandstone organic remains are so very rare, that the ex- 
istence of any, of whatever kind, in it was atone time denied by 
geologists. In the Lias first make their appearance the Ichthyo- 
sauri and Plesiosauri, large reptiles bearing some resemblance to 
the alligator, but furnished with paddles instead of legs and feet, 
as instruments of motion. Extending upward through the strata 
to the chalk, they have with the turtles, crocodiles, and other 
lizards that are associated with them, procured for this, the name 
of the saurian period in the history of the earth, or the age of 
reptiles. The Oolite abounds in organic remains consisting of 
corals and other shells, the reptiles of the Lias, and several dif- 
ferent kinds of Pterodactyle ; a creature in which were united the 
characters of a reptile, a bird, and a mammal, with wings like a 
bat, and supposed like him to have been abroad in search of food 
in the dusk of the evening or at night. Here also for the first 
time do we find the remains of a quadruped inhabiting the land. 
They are the bones of a species of Opossum. 
In the strata still above, other remains of other genera occur, 
but traces of land animals are either few in number or altogether 
wanting. Chalk which was long regarded as a precipitate from 
water highly charged with carbonate of lime, proves under the 
microscope, to be an aggregation of shells too minute to be distin- 
guishable by the naked eye. It is not till we come to the most 
recent strata and a bed of clay, sand, and gravel, covering 
all the other formations, that we find evidence that the earth 
has at a former, distant era, been thickly peopled by quadrupeds 
resembling those which now occupy its surface, though differing 
from them in some respects. It is in this upper bed that the fos- 
sil bones of Elephants, Hippopotami, and Rhinoceri occur. 
From these facts it may be inferred, that these different strata 
were formed in succession, and that each in its turn has been up- 
