50 J. D. Dana on the Minerals of Trap 



chytes and porphyries, which consist essentially of felspar, 

 have proceeded, in many instances at least, from felspathic 

 granites; the basalts and trap from syenites, hornblende or 

 augitic rocks. 



A theory proposed by Von Buch supposes that the fel- 

 spathic rocks, as they are of less specific gravity, are from the 

 earliest eruptions, or the more superficial fusings, while the 

 heavier basalt has come from greater depths. Darwin thus 

 accounts for the granites of the surface being intersected by 

 basaltic dykes ; the latter having originated from a deeper 

 source, where their constituents took their place at some 

 former period from their superior gravity. It virtually places 

 hornblende rocks below felspathic granites in the interior 

 structure of our globe. The hypothesis is ingenious and de- 

 mands consideration ; but it may not be time to give it our 

 full confidence. 



But supposing these more modern rocks to have been de- 

 rived from the more ancient granitic — what has become of the 

 quartz and mica which occur so abundantly in the latter, 

 while they are so uncommon in the former? By what changes 

 have they disappeared? 



In the fusion produced by internal fires, the elements are 

 free to move and enter into any combinations that may be 

 favoured by their affinities. If silica, alumina, magnesia, lime, 

 iron, the alkalies, potash and soda, were fused together — and 

 these are the actual constituents of basalt — what result might 

 we expect? From known facts, we should conclude that the 

 silica would combine with the different bases, and these simple 

 silicates would unite into more complex compounds. The 

 silicates of alumina and the alkalies or lime, form thus one set 

 of compounds, the felspars ; the silicates of magnesia and the 

 isomorphous bases, iron and lime, another set, to which be- 

 long augite, hornblende and chrysolite; and if much iron is 

 present, we might have with the lime and alumina, the mine- 

 ral epidote. The experiments of Berthier, Mitscherlich and 

 Rose, and the facts observed amongst furnace slags, confirm 

 what is here stated. 



But not to go back to a resolution of the fused minerals into 

 their elements, we may consider for a moment what changes 

 the minerals themselves might more directly undergo in the 

 process of fusion. 



Much of the mica in granite differs from felspar in con- 

 taining half the amount of silica in proportion to the bases, 

 the bases in each being alumina and potash or soda. The 

 change then in the conversion of the mica into felspar would 

 require an addition of silica, which might be derived from the 



