208 SILICATES OF LIME IN THE TRAP- ROCKS. 



solve the alkaline carbonates so slowly formed, iske up also a portion of 

 the silica, and convey it in a state of solution to the soils or to the rivers. 

 Ti)us, with the exception of the dews and rains which fall directly from 

 the heavens, few of the suppHes of water by which plants are refreshed 

 and fed, ever rea^h their roots entirely free from silica, in a form in 

 which it can readily enter into their roots, and be appropriated to their 

 nourishment. 



In the farm-yard and the compost-heap, wl^ere vegetable matters are 

 undergoing decomposition, the silicates they contain undergo similar de- 

 compositions, and, by similar chemical change* their silica is rendered 

 soluble, and thus fitted, when mixed with the soil, again to minister to 

 the wants and to aid the growth of new races of living vegetables. 



4°. Silicates of Lime. — A mixture of sand or flint with quick-lime 

 readily melts and forms a glassy silicate or a mixture of two or more 

 silicates of lime. These siHcates are also present in large quantity in 

 window and plate-glass, and in some of the crystalline* (granite and 

 trap) rocks. In felspar and mica, which abound, as we have seen, in 

 the alkaline siHcates, it is rare that any lime can be detected. In that 

 variety of granite, however, to which the name of syenite is given by 

 mineralogists, hornhlende takes the place of mica, and some varieties of 

 this hornblende contain from 20 to 35 per cent, of silicate of lime. This 

 silicate (containing 38 per cent, of lime) is almost always presen^ in the 

 basaltic and trap-rocks, and sometimes, as in the augiticf traps, in a 

 proportion much larger than that in which it exists in the unmixed horn- 

 blende. To this fact we shall have occasion to revert when we come 

 to consider the relative fertility of different soils and the causes on which 

 the ditTerence of their several productive powers most probably depends. 



Silicates of lime are also found in the ash, and probably^ exist in the 

 living stem and leaves of plants. 



Like the similar compounds of potash and soda, the silicates of lime 

 are slowly decomposed by the united agency of the moisture and" the 

 carbonic acid of the atmosphere. Carbonate of lime is formed, and 

 silica is set at liberty. This carbonate of lime dissolves in the rains or 

 dews which descend loaded with carbonic acid, [see page 46,] and the 

 same waters take up also a portion of the soluble silica and diffuse both 

 substances uniformly through the soil in which the decomposition takes 

 place, or bear them from the higher grounds to the fivers and plains. 

 The sparing but constant and long-continued supply of lime thus af- 

 forded to soils which rest upon decayed trap, or which ar^ wholly made 

 up of rotten rock, has a material influence upon their well-known agri- 

 cultural capabilities. 



5°. Silicates of Magnesia. — In combination with magnesia in differ- 

 ent proportions, silica forms nearly the entire mass of those common 

 minerals known by the names of serpentine and talc. In hornblende 

 also and augite, silicates of magnesia exist in considerable quantity. 



* So called because the minerals of which they consist are generally in a. cryatallized state 

 t Rocks of which the mineral called augite forms a more or less considerable part. 

 1 1 Sdiy probably, because if uncombined silica be present in hay or straw along with cai 

 bonate or oxalate of lime, the heat employed in completely burning away the organic mattei 

 may be sufficient to cause the lime and silica to unite and form a silicate wliicli will after- 

 wards be found in the ash, though none previously existed in the stem. 



