ORIGIN OF THE COMMON WATER-FORMED ROCKS. 49 



individual coral growths are mostly small, and as in a coral reef are 

 usually mixed with abundant fragments of worn corals and other 

 calcareous masses; all are commonly bound into a solid mass by 

 carbonate of lime deposited from the water soaking through the rock. 



Shell Limestone. One of the best English illustrations of a shell 

 limestone in process of formation in shallow water, is seen at Shell 

 Ness at the east of the Isle of Sheppey, in the Thames. There shells 

 are cast up so as to form a considerable level deposit, which in time 

 may well become cemented into a stratum like many of the shell beds 

 in the Carboniferous limestone and Lower Oolites. In some forma- 

 tions, like the Headon series in the north-west of the Isle of Wight, 

 there are considerable oyster-beds which have continued to grow and 

 accumulate although the mineral character of sediments around them 

 has changed several times ; just as oyster-banks grow at the present day 

 on the floor of the English Channel regardless of changing currents. 



Other limestones are almost entirely of vegetable origin. On certain 

 shores, especially in the tropics, plants called nuUipores grow, which, by 

 absorbing carbonic acid from the water, have the power of precipitating 

 around their tissues a dense coating of carbonate of lime. They are slender, 

 stony-looking, jointed tufts, termed corallines, and abundant on our own 

 shores, but with the joints growing to the size of fingers on coral reefs, 

 to the building up of which they contribute a not inconsiderable fraction. 



Freshwater Limestones, &c. In our own fresh waters there are 

 plants of the genus Chara, which thrive wherever waters contain much 

 lime, and possess a similar property of separating it from the water of 

 the pond, river, or lake, to that exhibited by the NuUipores. The stem 

 of the plant becomes coated with an incrustation, which ultimately 

 bears the plant to the bottom of the lake or stream, and so contributes 

 to build up a bed of limestone. Many of the limestones in the fresh- 

 water strata of the Isle of Wight, have originated in this way. Lime- 

 stones generally form ridges of more or less rounded hills, intersected 

 with deep valleys, which have been dissolved by rain water as it has 

 drained over the surface. The rounded contours of chalk hills are an 

 excellent example of the scenery produced in this way. Usually lime- 

 stone hills have but little wood growing upon them. See p. 105. 



Simultaneous Origin of Water-formed Rocks. These several 

 kinds of water-formed rocks are all forming at the present day in 

 lakes and on different parts of the sea-bed; and in all past ages 

 rocks consisting of these different mineral materials have accumu- 

 lated simultaneously in different regions ; so that a formation which 

 is clay in England may perhaps be sand in France, or a sand- 

 stone in this country may be represented by a limestone in Germany. 

 We shall the more readily understand how this is possible if we 

 imagine a coast or an island in process of being worn away by the sea, 

 and suppose it to be composed entirely of some crystalline rock such 

 as granite. This rock consists of three minerals, named quartz, ortho- 

 clase felspar, and mica, as already described. Speaking roughly, these 

 minerals are combined in granite in the proportions 25 per cent, of 



rtz, 55 per cent, of felspar, and 20 per cent, of mica. The quartz 



dirty-looking half-transparent mineral, in relatively large particles 



D 



