204 THE BOOK OF NATURE STUDY 



saurs (Fig. 28) in the sea, dinosaurs on the land, and pterodactyls in 

 the air. Loop-bearing brachiopods compete with abundant lamelli- 

 branchs and gastropods, and many of the limestones are coral reefs 

 abounding in sea urchins and a few genera of crinoids. Certain 

 shallow-water or terrestrial beds yield the remains of small 

 mammals, and an important vegetation consisting of ferns, 

 equisetums, conifers, cycads, and monocotyledons. The fishes are 

 still cartilaginous. Remains of the earliest known fossil bird 

 found at Solenhofen show that it had wing and tail feathers, but 

 was closely related to reptiles in the form of its tail, the shape 

 of its skull, the possession of four digits in addition to the " wing 

 finger/' and in having jaws furnished with teeth instead of a 

 beak. 



In addition to the economic products mentioned above, the 

 rocks furnish lime and hydraulic cement, alum, jet, roofing slabs, 

 iron-ore, sand, and water. There is even inferior coal in shallow- 

 water beds of this age in Yorkshire (Fig. 12). The scarped wolds 

 of the limestones with intervening clay vales sweep north and 

 southward from the Cotswolds (Figs. 52 and 63), and form a bar 

 which must be crossed by all roads and railways running from 

 London towards the north or west. 



The Cretaceous System was so named because the Chalk 

 Formation was one of its chief members. The other divisions 

 consist of alternations of clay and sand with some conglomerates : 

 In South Britain the lower beds are of fresh-water origin, and are 

 well developed in the Weald (Figs. 50 and 65) : The higher beds, 

 called " Greensands," are marine, and usually contain green 

 specks which are casts of the interior of foraminifera. The 

 Chalk itself is a pure white limestone composed chiefly of 

 foraminifera and shell particles, and it seems to have been laid 

 down in a fairly deep sea, as it has much in common with the 

 foraminiferal oozes of modern oceans. It carries bands or nodules 

 of flint, which are concretions of silica generally aggregated 

 round skeletons of sponges. Again, reptiles and ammonites 

 (Fig. 79) of various forms are conspicuous fossils, the former 

 including turtles and snakes, associated with extinct orders. 

 Mammals have not been found in Europe, but both mammals 

 and birds occur in America, the latter still possessing the reptilian 



