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THE NATURE BOOK 



Eozoic, which means dawn of hfe, from 

 the Greek eos, dawn, and zoe, hfe — and 

 of the hfe of this period we have still a 

 good deal to learn ; to the second era 

 the name Palaeozoic, meaning ancient 

 hfe, has been given, and is derived from 

 the Greek word palaios, ancient ; the 

 third era is called Neozoic, which means 

 new life, from the Greek neos, new, and 

 zoe, life ; and emphasizes the dominant 

 character of the forms of life represented 



Tlie Archaean or pre-Cambrian rocks, 

 forming the Eozoic group, are the oldest 

 known rocks in Britain, and are now 

 only naturally exposed to view in a few 

 localities where they originally formed 

 very high land, or have become exposed 

 by excessive denudation or great faulting ; 

 but in most places they are very deeply 

 covered by the deposits of the newer 

 formations. They have undergone con- 

 siderable changes, and are still a some- 



SHARK'S TEETH, FROM THE CHALK. 



by the fossils. To further facilitate the 

 geological record of the history of the 

 earth, each of these great eras is divided 

 into successive periods, while the rocks 

 peculiar to each period comprise a system. 

 During each period shght geographical 

 changes, accompanied by the change or 

 modification of the existing forms of life, 

 took place, and these are used to indicate 

 the limit of an epoch ; while the great 

 rock-masses, containing the physical and 

 life-history of the era, form a group of 

 strata. Thus the Neozoic era in Britain 

 is divided into two groups of strata, one 

 called the Cainozoic or Tertiary, the other 

 called the Mesozoic or Secondary. The 

 Cainozoic group comprises five geological 

 periods, while the Mesozoic comprises 

 three, and these again are divided into 

 epochs. The Pahecjzoic era is divided 

 into two groups — the Newer and Older 

 Palaeozoic, each comprising three periods ; 

 while the Eozoic has one period — the 

 Archaean. 



FOSSIL SPONGE IN FLINT CHALK. 



what ill-defined group. Of the rocks 

 forming this group, the Gneiss is probably 

 the oldest, and it presents the same 

 general characteristics wherever found, 

 although it is not all of the same age, some 

 probably being, comparatively, more 

 recent. According to Professor Bonney, 

 " Gneiss may be, if not actuaUy part of 

 the primitive crust of the earth, masses 

 extruded at a time when molten rock 

 could be reached everywhere near to the 

 surface." Between the Gneiss and the 

 unmistakably sedimentary rocks fre- 

 quently lie enormous masses of Crystalline 

 schists, sometimes several thousand feet 

 in thickness. No fossils having been 

 found in them, their origin is often difficult 

 and sometimes impossible to determine, 

 and but little has been discovered about 

 them. The upper schists probably re])re- 

 sent ancient limestones, clays and 

 sandstones ; while the lower Crystalline 

 schists are generally intensely folded and 

 crumpled, and are probably of igneous 



