STRUCTURE OF THE CRUST. 



The remains of one of these sea-worms, Nereites Cambrensis, 

 found at Llampeter, is shown in fig. 5. The body of this 

 creature consisted of about 120 joints. 



. 37. In line, then, we find that upon the igneous rocks as a 

 foundation the superficial structure of the earth has been erected, 

 consisting of a series of layers or courses of natural masonry, 

 one placed above another the formation of each of which has 

 been the work of countless ages ; that transition-rocks were the 

 first and earliest of these, while those which form the surface of 

 the earth, and are the habitation of the existing organised tribes, 

 were the last ; and that the epoch at which these latter tribes, 

 including the human race, were called into existence, remote as it 

 must appear, compared with all measures of time familiar to us, 

 is recent when referred to that system of chronology which is 

 written upon the crust of the globe. 



38. Above the transition-rocks, which, as we have stated, were 

 first placed in the class of primitive rocks, succeed a series of 

 layers which have been denominated SECONDARY BOCKS. These 

 consist chiefly of chalk, clay, argillaceous slate, shale, red and 

 brown sandstone, limestone, iron and lead ore, and coal. They 

 abound in organic remains, animal and vegetable, in a high state of 

 preservation, the minutest parts being often perfectly observable. 



39. The extent to which the earth was the theatre of organic 

 life, at the epochs of the deposition of these numerous strata, may 

 be conceived when it is stated that, in 1834, a German naturalist 

 and geologist counted no less than 9000 species, the remains of 

 which, at that date, had been found below the superior limits of 

 the stratified rocks, not one of which has ever existed since the 

 earth became the habitation of man. 



Among the animal remains which abound in these secondary 

 strata may be mentioned corals, crinoides, mussels, trilobites, 

 fishes, reptiles, insects, marine and fresh-water shells, sponges, 

 and animalcules countless in number. 



Of the reptiles, the most remarkable are various species of 

 lizard-shaped animals, constructed on a scale of colossal mag- 

 nitude, called Saurians, from the Greek word 2avpos (Sauros), 

 lizard. These have been variously denominated megalosaurus, 

 plesiosaurus, ichthyosaurus, and so on. 



40. Upon the secondary rocks repose a series of strata of more 

 recent deposition, called on that account TERTIARY ROCKS. These 

 consist of a thick bed of clay, limestone, sand, pebbles, and white 

 sandstone. They abound in organic remains, which are dis- 

 tinguished from those of the lower and more ancient strata by 

 including a considerable proportion of the still living species. 

 Thus the lowest strata of the tertiary rocks contained 5 per cent.? 



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