196 Mr J. D. Dana on the Origin of Trap Minerals, 



Essential constituents of modern Flutonic Bocks. — It is ob- 

 vious that modern igneous rocks, although in some cases de- 

 rived from the original material of the globe, have proceeded 

 to a great extent from a simple fusion of rocks previously 

 existing, and especially of the older igneous rocks. In ac- 

 cordance with this view, we may with reason infer that the 

 trachytes and porphyries, which consist essentially of feld- 

 spar, have proceeded, in many instances at least, from feld- 

 spathic granites ; the basalts and trap from syenites, horn- 

 blende or augitic rocks. 



A theory proposed by Von Buch supposes that the feld- 

 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 inter- 

 sected 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 feldspathic granites in the in- 

 terior structure of our globe. The hypothesis is ingenious, 

 and demands 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 afiinities. If silica, alumina, magnesia, 

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

 — and these are the actual constituents of basalt — what re- 

 sult might we expect 1 From known facts, we should con- 

 clude 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 feldspars : the 

 silicates of magnesia and the isomorphous bases, iron and 

 lime, another set, to which belong augite, hornblende, and 



