266 The Realm of Nature CHAP. 



MIOCENE Tropical plants. Ape, Antelope. 



OLIGOCENE Tropical plants. 



EOCENE Tropical plants. Palceotheriiim, Lemur. 



SECONDARY 



CRETACEOUS Flowering plants. Foraminifera, Marsupials, 



Toothed Birds. 

 JURASSIC Ferns. Saurians, Marsupials, Archaopteryx, Corals, 



Ammonites, Cuttlefish. 

 TRIASSIC Cycads. Ammonites, Reptiles. 



PRIMARY 



PERM IAN Amphibians. 



CARBONIFEROUS Lycopods. Tree-ferns, Conifers, Crinoids, 



Fishes, Amphibians. 

 DEVONIAN AND OLD RED SANDSTONE Fishes, Brachiopods, 



Lycopods. 



SILURIAN Sea-weeds. Graptolites, Trilobites, Fishes. 

 CAMBRIAN Trilobites, Sponges. 

 ARCHAEAN No forms of life known with certainty. 



346. Older Primary Rocks. The primary division is 

 called the Palceozoic, as in it the fossils of the earliest living 

 creatures are preserved. The Archaean, which forms the 

 foundation rocks, consists mainly of crystalline schists. 

 Wherever these appear on the surface we know that the 

 land is of extreme antiquity, for it must either have remained 

 above the sea while all the other formations were being 

 deposited elsewhere, or if it was upheaved after being 

 covered with younger rocks, the period must yet be suffi- 

 ciently remote to have allowed all the more recent strata to 

 be eroded away. No fossils are known with certainty in 

 Archaean rocks. The Cambrian, Silurian, and Devonian 

 systems, named after the districts in south-western Britain 

 where they were first studied, were formed in successive 

 periods. Fossils of sea creatures are abundant in these 

 rocks ; a peculiar crustacean called the trilobite swarmed in 

 the Silurian seas, and seems to have become altogether 

 extinct before the end of the Primary period. The earliest 

 land-plants, which were cryptogams, leave a record in the 

 Upper Silurian rocks. In the Old Red Sandstone rocks 



