138 STRATIFIED AND TRAP ROCKS. 



The. divisions of the strata of the earth are then, the primary, the 

 oldest and lowest rocks ; the secondary, the rocks which lie over the 

 last, and the tertiary, those which are uppermost and of the most re- 

 cent formation ; the strata of these divisions are sub-divided into 

 groups or formations. Some of the last, like the new red sand-stone, 

 afibrd a soil generally productive, while others, like the coal measures 

 and mill-stone grits, are generally unproductive ; but where two 

 soils meet, as with the lime-stone and clay, the soil is generally 

 improved. 



The stratified rocks occupy much the largest portion of the earth, 

 and form by far the greatest variety of soils. The unstratified rocks 

 (called also crystaline from their having a glossy appearance and crys- 

 tals within them, and likewise igneous rocks because they appear to 

 have been produced by fire,) are of two kinds, granite and trap rocks. 

 The first consist of a mixture of three minerals known as quartz, 

 felspar, and mica, principally of the two first. Quartz constitutes 

 from one-third to a half of the whole, and forms the substance silica, 

 before noticed. When the granite decays it forms the silicious sand 

 of soils. Felspar, when it decays, forms a very fine clay. Hills and 

 ridges of mountains are formed of granite. Rains and streams wash 

 out the felspar and carry it off in fine clay, while the quartz-sand re- 

 mains on the side-hills. The soil, therefore, on the flats and valleys 

 near granite mountains, are generally cold, stiff and wet clays, and 

 that on the side-hills is thin, sandy and barren. Granite rocks most 

 washed, have the light clay much washed out and are therefore most 

 barren. 



The trap rocks, consist of the green-stones and basalts, which 

 are composed chiefly of felspar and hornblende. Both of these are 

 reduced by the action of the weather to a powder affording the mate- 

 rials of soils. The principal earthy matter of the granite is felspar. 

 The chemical composition of felspar and hornblende greatly vary in 

 soils, as follows : 



Felspar. Hornbl. Hornblende. 



Silica, - 65 42 Magnesia, iJ,V - - 14 



Alumina, - 18 14 Oxide of Iron, - . - 14i 

 Pot-ash and soda, 17 Manganese, 1 



Lime, 12 



This difference must effect the soils they produce. A granite soil, 

 besides the silicious sand, consists mostly of silica, alumina and pot- 

 ash ; and a hornblende soil consists, in addition to silica, of lime, 

 magnesia, and oxide of iron, in the proportions of 2 cwt. of each for 

 every ton of the decayed rock. A hornblende soil therefore contains 

 more of the constituents of plants than that of felspar. But these 

 being mixed, as in the greenstones, the soil is still more productive ; 

 the felspar supplying the soda and pot-ash, which the hornblende 



