GEOLOGY. 561 



in its strata small yellowish sub-globular concretions, re- 

 sembling in their appearance fish-eggs, which has procured 

 for it the name of Oolite. 



The great feature of this period is the first appearance of 

 mammals. The only vestiges found of them are two little 

 jaws, belonging to species very like the opossum, so well 

 known from the habit of the female of carrying her young 

 family in an abdominal sack, or bearing them on her back. 1 



The oolite abounds in molluscs, polypoids, and fossil plants. 

 Insects and crustaceans are also found in it. 



The last group of the secondary strata, the cretaceous or 

 chalk formation, plays an important part in geology : partly 

 owing to its depth, partly to the great extent over which 

 it is found. It is scarcely necessary to say that it owes its 

 name to the chalk (carbonate of lime), of which it almost 

 entirely consists. The cretaceous strata form many of our 

 mountain chains. 



During this period both land and sea appear to have been 

 still under the domination of reptiles of colossal size. The 

 Mosasaurus, long called the " great Maestricht animal," an 

 immense marine lizard, attained a length of more than 

 sixty-five feet, whilst contemporary species are not more 

 than a yard long. It must have spread terror on all sides. 



1 The oolite, which produced the famous lithographic slate of Solenhofen, 

 yielded the first bird, the skeleton of which has been so far preserved that its 

 nature could be clearly decided upon. This is the Archaeopteryx, now in the 

 British Museum. It exhibits a closer approximation to reptilian structure than 

 any modern bird. The tail is very long, and in this respect more like that of a 

 reptile than that of a bird. Two digits of the manus have curved claws, much 

 stronger than those of any existing bird. Popular Science Review, vol. vii., p. 

 241. TR. 



