364 Prof. J. D. Dana on some Results of 



being capable of producing an increase of density, and at some 

 depth that density which belongs to the solid rock ; so that 

 downward either from the plane at this depth, or from some 

 level or levels below it, actual solidity would have existed. It 

 may be that when exterior solidification (that is, the solidifica- 

 tion of the crust) was about to begin, the outer limit of the in- 

 terior solid mass under the solidifying areas was quite up to the 

 spherical plane in which the rock of the interior had the density 

 of solidity. It is at least certain that its limit was as near the 

 surface as was possible under the temperature then existing. 



3. Nature of the cooling crust, and of the liquid layer of which it 

 was formed. 



The change of specific gravity or density which the rock- 

 material underwent in passing from the liquid to the solid state 

 is part of the data required for conclusions on the subject in 

 view ; and with reference to it we may first consider the nature 

 of the rock-material. 



The larger part of the igneous rocks of the globe are ejections, 

 according to the evidence which has been presented, either from 

 regions within the true crust of the earth (that is, the part situ- 

 ated below the supercrust and which was a direct result of the 

 cooling), or from the plastic layer or seas underneath. In either 

 case they are testimony, and have long been so regarded, with 

 respect to the outer liquid layer of the melted sphere. The 

 next best testimony we have is that from the earlier of the sur- 

 face formations — the Archaean (Azoic). Another source of con- 

 clusions, appealed to effectually by Daubree*, is the constitution 

 of meteorites, and parallel facts in the earth's igneous and meta- 

 morphic rocks. Still another, used by Hunt, is reasoning from 

 physical and chemical laws to the probable results under the 

 supposed conditions of a cooling globe. We thus arrive at the 

 following conclusions. 



(a) The more prominent minerals were the following : — (1) some 

 iron-bearing species (bearing also magnesia and lime) of the 

 amphibole family, as augite, hornblende ; (2) the iron-and-mag- 

 nesia mineral chrysolite (or olivine), to which Daubree, on the 

 evidence just referred to, gives great prominence; (3) species 

 of felspar — either the lime felspar called anorthite, the lime-and- 

 soda kind called Labradorite, the soda-and-lime kind oligoclase 

 (and andesine), the soda felspar (containing usually some potash) 

 albite 3 the potash felspar (containing usually some soda) ortho- 



* "Experiences synthetiques relatives aux Meteorites. Rapproche- 

 ments auxquels elles conduisent, tant pour la formation de ces corps pla- 

 netaires que pour celle du globe terrestre," par M, Daubree, Comptes 

 Rendus, vol. lxii. (1866). 



