92 THE BUILDING OF AN ISLAND. 



a few of the fir familv, but most of the trees, as well as the smaller plants, 

 belonged to the fern family and other flowerless plants. The shales which 

 cover the coal frequentl)^ contain fragments of the same plants mixed with 

 sea shells, which shows that the jungle in which the plants grew was over- 

 whelmed by an invasion of the sea, the result presumably of the sinking of the 

 land, a movement which must have taken place over and over again, with long 

 periods of rest between, before the numerous beds of coal and the many inter- 

 vening sandstones, clays and so on, of which the coal measures are made up, 

 could have been produced. 



In all ages of the earth's history the outline of the shore, the different 

 depths in the adjoining sea, the courses of the great rivers, and the like, in 

 short the physical geography of the time and the region, must have had a 

 large share in determining the nature and extent of the respective deposits. 

 Hefice we find in the English Carboniferous system that its above named 

 three divisions are differently developed in the different coal-fields. In the 

 north the Mountain Limestones and the Mill-stone Grits are verv' largely 

 developed, in South Wales the Coal Measures, which there attain a thickness 

 of about I i,ooo feet. The separation into "fields " is the result of movements 

 which have taken place since the strata were deposited. The elevation of the 

 Pennine range, for instance, has brought the Mountain Limestone to the top, 

 the higher formations having been all swept away, but having been left on the 

 Avest of the chain to form the coal-fields of Cumberland and Lancashire, and 

 oii the east to form those of Northumberland, Durham and Yorkshire. The 

 above short sketch of the English Carboniferous system may serve to show 

 what is meant by a geological system; but it should be added that the English 

 coal-bearing strata, extensive as they are and important as they are to the 

 country's prosperity, are yet only a small part of the world's wealth in this 

 particular, for many extensive coal-fields are found in parts of Europe, North 

 America, India, China, Australia and South Africa, the North American fields 

 in particular being very large and far exceeding in area those of England. 



In the Coal Measures the fossils naturallv belong mainly to the vegetable 

 kingdom, but there are also throughout the carboniferous system, as already 

 mentioned, many shells, corals and encrinites; there are also forminiferous 

 shells, insects, etc. Other systems are marked by special classes of fossils, for 

 example, the older rocks contain crablike creatures called trilobitcs, their shells 

 having ///r^^ lobes. The Old Red Sandstone of the north of England contains 

 many kinds of fossil fishes, while the rocks of Devon, belonging to the same 

 period, consist mainly of slates and limestones and contain numerous corals. 

 The Jurassic period is famous as the age of gigantic reptiles belonging to the 

 lizard family. There were snake-like lizards, flying lizards with bat-like wings 

 and fish-like lizards. The skeleton of one of these creatures now in the 

 Museum of Natural Historv in New York measures 60 feet in lensfth and 11 

 feet in height. In the time of the deposition of the Oolitic limestones, which 

 form part of this system, there were also numerous sea creatures resembling 



