THE WORLD BEFORE MAN 19 



Our third picture may represent the world of 

 what geologists call the archaean, or eozoic period, 

 when the crust had been furrowed up into ridges of 

 land,, and corresponding but wider depressions occu- 

 pied by the sea. Into the latter the rains falling on 

 the land are carrying sediment derived from the 

 wasting rocks, though the waters are still warm and 

 the thinner parts of the crust are still welling out 

 rocky material, either molten or dissolved in heated 

 water. In this period there were probably low forms 

 of animal life in the waters and plants on the land, 

 though we know little of their exact nature. 



A fourth picture may represent that great and 

 long-continued palaeozoic period in which the waters 

 swarmed with many forms of life, when fishes were 

 introduced into the sea, and when the land became 

 covered with dense forests of plants allied to the 

 modern club-mosses, ferns, mares'-tails and pines ; 

 while insects, scorpions and snails, and some of the 

 humbler forms of reptiles, found place on the land. 



Returning after an interval, we should see a fifth 

 picture, that of the mesozoic world. This was the 

 age of reptiles, when animals of that class attained 

 their highest and most gigantic forms, and occupied 

 in the sea, on the land, and in the air the places now 

 held by the mammals and the birds ; while the con- 

 tinents were covered with a flora distinct alike from 

 that of the previous and succeeding periods, replaced, 

 however, as time went on by forests very like those 

 of the modern world. In this age the earliest mam- 



B 2 



