340 ALTERNATIONS OF LANB AND WATER. 



the surface of the earth, the many bands of mountain 

 limestone were formed by the ceaseless activity of these 

 minute architects. Encrinites (creatures in some respects 

 resembling star-fish) existed in vast numbers in the 

 oceans of this time ; and the great variety of bivalve 

 shells, and those of a spiral character, discovered in the 

 rocks of this period, show the waters of the newer palse- 

 ozoic period to have been instinct with life. 



In the world then, as it does now, water acting on 

 the dry land produced remarkable changes. We have 

 evidence of extensive districts over which the most 

 luxuriant vegetation must have spread for ages, from 

 the remains of plants in every state of decay, which we 

 find went to form our great coal-fields. These, by some 

 changes in the relative levels of land and water, became 

 covered with this fluid ; and over this mass of decaying 

 organic matter, sand and mud were for ages being de- 

 posited. At length, rising above the surface, it becomes 

 covered with vegetation, which is, after a period, sub- 

 merged ; the same deposition of sand and mud again 

 takes place, it is once more fitted for vegetable growth, 

 and thus, cycle after cycle, we see the dry land and the 

 water changing places with each other. This will be 

 evident to every one who will carefully contemplate a 

 section of one of the coal-fields of Great Britain. We 

 find a stratum of coal lying upon a bed of under clay, and 

 above it an extensive stratum of shale or sandstone, pro- 

 bably formed by the denudation of the neighbouring 

 hills ; and in this manner we have many strata of coal, 

 shale, clay, ironstone, and sandstone alternating with 

 each other ; the coal-formations of the South Wales coal- 

 field having the extraordinary thickness of 1500 feet. 

 The lowest bed of this extensive series must at one time 

 have been exposed as the surface of the country. 



Ascending in the series, we have now formations of a 

 more recent character, in which fishes of a higher order 

 of organization, creeping and flying saurians, crocodiles 



