412 



THE GEOLOGIST. 



would go to form the lower strata of the globe, whilst the upper 

 might owe their origin both to the subsequent erosion of the first 

 strata deposited, and also to vast eruptions of volcanic matter from 

 the interior of the yet unsettled earth. The admission of the latter, 

 the volcanic agency, fully accounts for the great variety of mine- 

 rals and metals found deposited in rocks daring different ages, 

 whilst the varying character of lavas ejected in the same localities, 

 yet separated by long intervals of time, would in some measure show 

 the reasonableness of the supposition that the interior of the globe 

 is in reality the grand repository whence every mineral had its 

 origin.* 



Assuming the nebular theory to be correct, the following would 

 appear to be the sequence of changes occurring during the earth's for- 

 mation. 



All matter existing originally in the form of a gaseous mass, the 

 particles of which were kept separate by the contained caloric, on cool- 

 ing to a certain point, the least easily volatizable parts would assume 

 either a solid or a liquid form. Immediately on this taking place, the 

 particles so forming, would of necessity rush together by the force of 

 gravity, and must inevitably meet and form a solid or liquid nucleus to 

 a surrounding nebulous atmosphere. The contained heat of the nebu- 

 lous atmosphere still further decreasing, other solid or fluid matter 

 would be formed, and would be deposited in the form of a stratum 

 upon the solid or liquid portion first originated. In this way would 

 be built up, as it were, avast globe, whose materials would be arranged 

 in layers or strata, the order or position of which would be the order 

 in which they respectively solidified. t 



During the formation of such a globe, it might sometimes happen 

 that a stratum of a greater specific gravity would immediately suc- 

 ceed one of less. If the former chanced to remain a fluid, it is evi- 

 dent the two would be placed in a state of instable equilibrium, and 

 that the, at first, lower stratum would, by the inevitable laws of gra- 

 vity and hydrostatics, ultimately assume the higher position. This 

 state of things may be realized by imagining iron or other light metal 

 and mercury to be introduced, both in a state of vapour, into the same 

 receiver. If the lighter metal were the first to assume the solid 

 form, it would be naturally deposited upon the bottom of the con- 

 taining vessel, but would rise to and float upon the surface of the 

 mercury immediately on the tatter's condensation. 



Applying this theory to the formation of the globe, let us suppose 

 the granite at a given period to have been already precipitated, the 

 heat of the nebulous mass being diminished by a required amount ; and 

 upon this layer of granite to be subsequently thrown down a fluid of 



* In New Zealand there is said to be miles and miles of fine steel-dust covering the 

 seabeaeh to a considerable depth ; this dust is supposed to be ejected by volcanic action. 



f So far, most part of the above theory is identical with that of M. Ampere, noticed 

 in the ' Edinburgh Philosophical Journal,' vol. xviii. p. 339. The author, however, begs 

 to state that he never heard of M. Ampere's theory until long after his own had been 

 elaborated. 



