ORIGIN OF THE COMMON WATER-FORMED ROCKS. 45 



materials can be carried no farther, they are more frequently named 

 sedimentary deposits. Limestone does not accumulate as a sediment. 

 The carbonate of lime can only be taken up into the water when 

 water contains sufficient carbonic acid gas to dissolve it ; and water 

 only parts with it again when the surface layer is evaporated, or when 

 some plant or animal separates the lime from the water to form its 

 skeleton. Deposits due to these causes accumulate over the whole 

 bed of the ocean, though their rate of accumulation is so slow that 

 the lime is usually inappreciable near to shore where other deposits 

 are forming, except as a cement, binding sands or hardening clays. 

 These layers of sand, clay, and limestone extend through the country, 

 sometimes turned up from their original horizontal positions almost 

 on end, sometimes bent into basin- shaped folds, and sometimes folded 

 into saddle-shaped ridges. Pebbles are only formed where the rocks 

 on the shore or a shallow sea-bed are hard, and are worn away by the 

 pieces being slowly ground against each other, and thus become 

 rounded. They may consist of any kind of rock, and are usually 

 evidence of near vicinity to land at the time of their deposition. 

 Mr. Darwin mentions that at Santa Cruz, on the sloping east coast 

 of South America, the pebbles near to shore are very large, while at 

 three or four miles from the shore they are as large as walnuts ; at 

 six or seven miles, as large as hazel nuts ; at ten to eleven miles from 

 shore they were from f^ths to j^ths of an inch; at twelve miles, ^gths 

 of an inch ; and from twenty-two to one hundred and fifty miles, the 

 sediment varied in size from ^th of an inch to the finest sand. Over 

 this distance the depth steadily increases to sixty-five fathoms. Many 

 beds of pebbles occur along our own shores, especially where there are 

 chalk cliffs or granite, or other old and hard rocks to furnish materials 

 out of which they may be formed. According to Colonel Greenwood, 

 pebbles of chalk-flint are carried by the sea as far west as the south 

 of Cornwall, and pebbles of the Cornish rocks are mixed with the 

 flint pebbles of Sussex and Kent. There are many beds of conglo- 

 merate among the British strata, as will be seen by a glance at the 

 table given further on. 



Mr. Darwin's observations, just referred to, show that the pebbles 

 become smaller farther from shore. The small pebbles are chiefly 

 pieces broken from the larger masses, and often are the larger pebbles 

 worn small by constant rubbing against each other. When they are 

 no larger than peas, the rock they form is often called grit. The 

 British strata contain several such beds ; as, for instance, the Millstone 

 Grit, which lies below the formation named Coal Measures. 



Sand. The finer kind of sediment called sand consists of the 

 mineral quartz. This is either the fine dust ground from off flints or 

 old quartz rocks in the process of rounding pebbles, or is made up of 

 grains of quartz, which constitute part of some crystalline rocks such 

 as granite and the schists. These grains are usually bound together 

 with a cement of silica, or carbonate of lime, or oxide of iron, so as 

 to be hardened into stone. The most familiar example of sandstone 

 is seen in the paving-stones which form the footways in London 



