the Earth's Contraction from Cooling. 367 



likely to have been formed over the earth's surface after the 

 action on the crust of the foul atmospheric vapours that settled 

 upon it as it began to solidify. If there were mainly doleritic 

 material and other Labradorite mixtures in that crust, the 

 result of the conflict would be a removal of part of the bases 

 and the liberation of silica, making free quartz and quartz- 

 bearing rocks. 



Again, the general fact that the doleritic rocks, and even most 

 trachytic, contain disseminated grains of uncombined oxide of 

 iron in the form of magnetite (Fe 3 O 4 ) , adds to the strength of 

 the argument against the general diffusion of quartz (that is, 

 free or uncombined silica), if not proving its absence. 



(e) The presence of a large proportion of iron is a marked 

 feature of most igneous rocks. This ferriferous quality i3 not 

 characteristic solely of the doleritic and syenitic kinds ; even the 

 most purely felspathic trachytes usually contain some dissemi- 

 nated magnetite and hornblende ; and from this extreme there 

 is a shading-off in trachytic rocks toward dolerites, syenites, or 

 hyposyenites. It should be here understood that augite and 

 hornblende are essentially identical in chemical constitution, 

 though differing in crystallization. Hornblende has often a 

 slight excess of silica, making the oxygen ratio of the bases and 

 silica frequently 1 : 2|, as in the felspar oligoclase, instead of 

 1 : 2, as in augite ; and this may be a reason for its occurrence 

 by preference in the trachytes, in which oligoclase and ortho- 

 clase predominate, and that of augite in the dolerites, in which 

 Labradorite is the predominant felspar. In Labradorite this 

 ratio is 1 : 1J, and in Andesine 1 : 2. Mixtures of Labradorite 

 and oligoclase, which constitute the base of some doleritic rocks 

 (melaphyres, in which the silica constitutes over 55 per cent.), 

 would have 1 : 2 for this oxygen ratio when the proportion of 

 Labradorite to oligoclase was 1 to 2. 



The basic iron-bearing feature of the first solidified crust is 

 attested also by the nature of the lowest rocks of the super- 

 crust — that is, by the Archaean formations overlying the true 

 crust and directly or indirectly made from it. We find among 

 the Archaean terrains the felspar Labradorite far more abundant 

 than in any later metamorphic rocks, the rock hypersthenite 

 common (which is much like dolerite in elemental constitution), 

 others (ossipyte and a chrysolitic hypersthenite) which are closely 

 related to peridotite or chrysolitic dolerite, other rocks that are 

 almost solely chrysolite, another (diabase) which has the com- 

 position of a chloritic dolerite, other kinds (referred to hyper- 

 sthenite and diabase) which approach melaphyre. And besides, 

 there are diorite, consisting of hornblende and albite or oligo- 

 clase, and hvposyenite, consisting of hornblende and orthoclase. 



