136 DEPOSITS OF SUBSTANCES HELD IN SOLUTION. 



atid carry away different substances ; some they also dissolve, and 

 afterwards deposit them, by evaporation, in form of solid sediments, 

 which are sometimes more or less crystalline. To the infiltration 

 of these waters, for example, is due all kinds of stala'ctites (from 

 the Greek slalassb, I drop), which form in various subterraneous 

 cavities, and especially large in caverns found in calcareous coun- 

 tries. Certain waters are rich in dissolved materials, and suffi- 

 ciently abundant to give rise to extensive deposits on the surface 

 of the earth. Those particularly, which, by carbonic acid, hold 

 a great quantity of carbonate of lime in solution, and which, from 

 abundant or numerous springs, give origin to rivulets and even 

 lakes, at the bottom of which is daily formed what is called traver- 

 tin or calcareous tu'fa. These waters are met almost everywhere, 

 in calcareous regions. Scattered over a flat country, or on the slope 

 of a valley, these waters incrust the plants growing there, and, 

 from these agglomerated and superposed incrustations are formed 

 considerable rocks, the mass of which is consolidated by waters 

 which percolate the interstices they meet, and render the whole 

 solid and uniform. When these waters flow over slopes free from 

 vegetation, they deposit thin and successive layers, following the 

 undulations, the whole forming compact masses which daily grow 

 in thickness. In lakes into which waters of this kind fldw, hori- 

 zontal beds of solid calcareous matter are formed, which are often 

 filled with fluviatile, and even terrestrial shells, daily brought 

 into it. 



31. Sands washed up by waves, either in fresh-water lakes or 

 seas, are daily consolidated by waters more or less charged with 

 carbonate of lime. Examples of this kind are seen in the sands 

 of lake Superior, in those of the gulf of Messina, at several points 

 on the coasts of England, of the West-India islands, chiefly at 

 Guadaloupe, New Holland, &c. These arena'ceous substances 

 often become sufficiently solid for building purposes. 



32. Sili'cious deposits. A great many mineral waters, particu- 

 larly those which are warm or hot, contain, besides carbonate of 

 lime, a certain quantity of silex (from the Greek cha/is, a pebble) ; 

 on this account many calcareous tu'fas are more or less silicious. 

 But there are springs in which the silex is sufficiently abundant to 

 form considerable deposits of hydrated sili'cious deposits, some- 

 limes nearly pure, and sometimes mingled with other substances. 

 The tu'fas of the geyser in Iceland are deposited for nearly a 

 quarter of a league round the spring, three-quarters of a yard thick. 

 One of the^e geysers (a word which according to some means 

 spouting, and furious, according to others) spouts up every half 



30. How do waters form deposits from matter held in solution ? Whal 

 tre stala'ctites ? 



31. By what means are sands consolidated? 



32. How are sili'cious deposits formed ? What is a geyser ? 



