MANUAL OF AGPJCULTURE. 233 



reoiis sands compacted by rain, the debris at the bottom of cliffs, 

 and soil are examples ; 4th, The metamorphic, those which have 

 undergone change of texture since the eruption or deposit of their 

 constituents. Traces of orrranised remains be^in to be met with 

 in the earliest aqueous rocks, and thus attain a higher develop- 

 ment in proportion to our ascent to the latest aqueous deposits, — 

 stratifications, — till it becomes perfect as that of the living forms, 

 both animal and vegetable, now existing. The branch of geology 

 dealing with such traces, or fossils, as they are called, testifying 

 to the forms of life co-existent with the deposition of the specific 

 materials of the strata where they are found, is named Paleon- 

 tology — an abstruse study, its prosecution demanding the pre- 

 liminary of a highly scientific training. 



The igneous rocks principally exist as granite and trap. They 

 are chemical productions, -i.e., have been consolidated from fusion 

 by chemical means. Besides forming the solid framework of the 

 earth, and the foundation of the other rocks, they are upheaved, 

 and constitute the principal mountain chains, and they are 

 exposed in masses of enormous area. They are also poured out 

 in profusion as lava and scoriae during volcanic action, and they 

 permeate the rents and crevices of the sedimentary rocks. All 

 Igneous rocks are composed of minerals, silicates, to wit, i.e., salts 

 formed by the union of silicic acid with a base. These silicates are 

 divided into two classes — silicates of ma^'nesia and silicates of 

 alumina ; and the various subdivisions in each are constituted by 

 so many mixtures with silicates of potash, soda, lime, iron, 

 manganese, &c. The uncovered masses of igneous rock <<^ene- 

 rally being situated at high altitudes, the soils they form are at 

 such an elevation as to be incapable of cultivation, and they are 

 accordingly left in the natural condition. But when such soils 

 exist in practicable situations for the agriculturist, or the ddritvs 

 of them is conveyed thither, great fertility obtains, and the soil 

 is easily worked. This is specially predicable of soils derived 

 from trap rocks. 



jMetamorphic rocks are produced by the alterations eftected by 

 heat in the texture and structure, and by its rearrangement of 

 the atoms of the constituents of their originators. AlthouLih re- 

 suiting from the changes in strata of all epochs, still they, for the 

 most part, lie over or against the huge igneous masses, being com- 

 prehended in the so-called Laurentian, Cambrian, and Silurian 

 eras of formation. Together willi the igneous, they constitute the 

 principal i)art of wilder Wales and the Scotch Highlands ; and 

 whilst such tracts admit of little cultivation, they are admirably 

 suited for slice}) runs. 



The stratified rocks have Ix'cn jiroduced mechanically, as we 

 have seen, by the agency of the atmosphere and water ; chemi- 

 cally by the precipitation uf their constituents from solution in 



