42 SECRETS OF EARTH AND SEA 



remains of vertebrates are found. These first vertebrates 

 to appear (others soft and destructible preceded them) 

 are fishes — a group which, apart from this fact, are 

 shown by their structure to present the ancestral form 

 of all the vertebrate classes. In later Palaeozoic beds we 

 find the remains of four-legged creatures like our living 

 newts and salamanders. The Secondary or Mesozoic 

 series is divided into the Triassic, Jurassic and Cretaceous 

 systems. It ends with the familiar chalk deposit of this 

 part of the world, and is often called the age of Reptiles, 

 because large reptiles abounded in this period. The 

 Tertiary or Cainozoic series are divided into the Eocene, 

 Oligocene, Miocene, Pliocene and Pleistocene systems. 

 The huge reptiles disappear and their place is taken by an 

 endless variety of warm-blooded, hairy animals — the 

 Mammals — small at first, but in later beds often of great 

 size. As we pass upwards from the Eocene we can trace 

 the ancestry of our living Mammals such as the horse, 

 rhinoceros, pig and elephant in successive forms. Com- 

 plete skeletons are preserved in the rocks and show a 

 gradual transition from the more primitive Eocene kinds 

 — through Miocene and Pliocene modifications — until 

 in the Pleistocene strata many of the species now 

 inhabiting the earth's surface are found. A number 

 of horizons, characterized by the special mammalian 

 and other animal remans preserved in them, are dis- 

 tinguished by geologists in each of the " systems " 

 of sands, clays and harder beds known as Eocene, 

 Oligocene, Miocene and Pliocene. At last we arrive at 

 the latest or most recent 250 feet of deposit, consisting 

 of sand, clay and gravel. This is called " Pleistocene." 

 It is only a very small fraction (Vt^-th) of the thick- 

 ness of the whole fossil-bearing sedimentary crust of 

 the earth — about the proportion of the thickness of a 

 common paving-stone to the whole height of Shakespeare's 



