158 GEOLOGY. 



phuric, is added to oaze, brisk effervescence occurs, be- 

 cause the acid, taking the lime, sets the carbonic acid 

 gas free. Here there is an immense chalk deposit going 

 on over a large area ; and in such lengths of time as were 

 occupied by deposits and solidifications in ages gone by, 

 this deposit may become hundreds of feet thick, and be 

 solidified into rock. In limestone strata quarried near 

 Paris the rock is composed to a large extent of shells 

 no larger than millet-seeds. Ehrenberg has discovered 

 in that form of limestone which we call chalk, animals 

 much smaller than this so small that, where they nearly 

 constitute the whole mass, as they do in the chalk of 

 southern Europe, there are over# million of them in ev- 

 ery cubic inch. Soldani collected from less than an 

 ounce and a half of rock from the hills of Casciana, in 

 Tuscany, 10,454 shells of foraminifera of various specie^. 

 Some of the species are so minute that it would require 

 over a thousand of the shells to weigh a grain. 



240. Silicious Shells. You have already learned in 

 116 that silica, or flint, is dissolved by certain means in 

 water, and then is gathered up by plants and animals, 

 becoming again solid in soluble silica. There are very 

 extensively scattered in the earth innumerable minute 

 animals and vegetables which thus gather this material 

 from the water in which they live, and then, as they 

 die, their silicious shields or shells are deposited, and 

 much of the deposit becomes in time solid rock. These 

 shields present great variety of figure, and are very beau- 

 tiful as seen under the microscope. In Fig. 84 you see 



represented three 

 of the forms of 

 shells of this kind, 

 ^8fc -:/' of which a stratum 



Actinocydua. Coscinodiscus o f w hite clay about 



patina. 



Fig. 84. Richmond, Va., is 



chiefly composed. The stratum is from twelve to thir- 

 ty feet thick, and is of great extent. The tripoli, or rot- 



