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XII. On the Origin of the constituent and adventitious Mine- 

 rals of Trap and the allied Rocks. By James D. Dana*. 



THE minerals of trap and the allied rocks may be arranged 

 in two groups : — 



1. Those essential to the constitution of the rock, or inti- 

 mately disseminated through its texture. 



2. Those which constitute nodules or occupy seams or 

 cavities in these rocks. 



Of the first group, are the several felspars, with augite, 

 hornblende, epidote, chrysolite, leucite, specular, magnetic 

 and titanic iron; and occasionally Hauyne, sodalite, sphene, 

 mica, quartz, garnet and pyrites. Of the second group are 

 quartz, either crystallized or chalcedonic, the zeolites or hy- 

 drous silicates, Heulandite, Laumonite, stilbite, epistilbite, 

 natrolite, scolecite, mesole, Thomsonite, Phillipsite, Brew- 

 sterite, harmotome, analcime, chabazite, dysclasite, pectolite, 

 apophyllite, prehnite, datholite, together with spathic iron, 

 calc-spar and chlorite. Native copper and native silver might 

 be added to both groups, yet they belong more properly to 

 the latter. To the same also might be added sulphur, and 

 the various salts that are known to proceed from decomposi- 

 tions about active volcanoes, including the crystallizations of 

 alum, gypsum, strontian, 8cc. ; but these more properly form 

 still a third group, and being well understood, will not come 

 under consideration in the remarks which follow. 



We observe with regard to the minerals of the first group, 

 that they are all anhydrous, that is, contain no water. In 

 this respect, the essential constituents of trap and basalt are 

 like those of granite and syenite. But in the second group, 

 consisting of the minerals occurring in cavities or seams, all 

 contain water except pectolite, quartz, calc-spar and spathic 

 iron ; and the last three are known to be always deposited in 

 an anhydrous state from aqueous solutions. 



We proceed to give a few brief hints with regard to the 

 first group, intending only to glance at this branch of the sub- 

 ject, and then take up more at length the group of adventi- 

 tious minerals. 



Essential constituents of modem Plutonic rocks. — 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 ex- 

 isting, and especially of the older igneous rocks. In accord- 

 ance with this view, we may with reason infer that the tra- 



* Read before the Association of American Geologists and Naturalists, 

 May, 1 845, and communicated by the Author. 

 Phil. Mag. S. 3. Vol. 28. No. 1 84. Jan. 1846. E 



