232 MANUAL OF AGEICULTURE. 



when the earth had commenced its career of revohition around 

 the sun, its consistence would be that of a pasty mass, enveloped 

 with dense vapours and gases. Its present spheroidal shape 

 would ensue upon its rapid revolution upon its own axis. With 

 the process of cooling down would keep pace that of the conden- 

 sation of its surrounding fogs and gases into air and water^ 

 whilst its more solid mass would concentrate in bulk, pressing- 

 inwards towards the centre of gravity with the attendant effect of 

 irregularity of surface, caused by the absence of a uniform degree 

 of internal resistance to such external pressure. With tlie 

 increased intensity of this pressure, more marked would become- 

 mountain and valley. Moreover, the resisting force of the inter- 

 nally confined fluid substance would gradually prevail over th& 

 weaker portions of the crust, and its upheaval, ^vitli all the 

 violence of earthquake and volcanic convulsion, would vastly 

 exaggerate the superficial irregularity of Moses' "dry land.'* 

 Thus did it appear above the face of the waters, but naked as 

 yet, — sterile, vdthout soil, entirely devoid of living organisation. 

 Then gradually atmospheric action would crumble down those 

 bare rocks exposed to its action. The detritus washed into the sur- 

 rounding depth of waters, there subjected to the superincumbent 

 23ressure for epochs of time, became solid strata or layers, thence 

 to be upheaved and exposed to the atmospheric process, as a rock- 

 forming- material different in nature and texture from its ov'vA- 

 nators. And so on, the alternate depression and upheaval con- 

 tinues even to the present time,butf'in an infinitesimally less degree, 

 for the cover of vegetation over the larger portion of the earth's 

 surface protects from the erosive action of the atmosphere, and 

 earthquakes are less frequent, and the more considerable volca- 

 noes for the most part inert. By the earth's crust is meant the 

 thickness of it which has come under the cognisance of cjeolo- 

 gists, and it bears an inappreciable proportion, of course, to the 

 earth's diameter. Heat increases in the ratio of our depth of 

 penetration through the crust, proving the immensity of heat still 

 present within our globe, which is conjectured internally to be 

 in a molten state, or at least in a honev-combed condition, with 

 molten matter filling the cells. This is evidenced by the pheno- 

 mena of lava and hot springs, even more strikiugly than by the 

 large increase of temperature in deep artificial mines. The rocks^ 

 — and this term includes clay, gravel, and sand deposits, — which 

 compose this crust, are divided into — 1st, The igneous, being those 

 formed by the agency of fire, or from fused melted matter, and 

 preserving their original condition ; 2d, The aqueous or sedi- 

 mentary, comprising such as have been formed by the deposit of 

 detritus of rocks exposed to the air, and laid down under the 

 water in regular strata ; 3d, The aerial, or such deposits as have 

 been accumulated by atmospheric agencies. Sand dunes, calca- 



