62 J. D. Dana on the Minerals of Trap 



These few hints are barely sufficient to indicate something 

 of the interest that attaches to this field of investigation, which 

 the future developments of science will probably open fully 

 to view. We do not attempt to explain why in these modern 

 fusings, mica should not have remained mica, and the quartz 

 still free uncombined quartz. The facts prove some peculi- 

 arity of condition attending the formation of the granitic rocks. 

 Of this condition we know nothing certain, and can only 

 suggest the common supposition of a higher heat and slower 

 cooling, attending a greater pressure and different electrical 

 conditions, and the same circumstances may have existed 

 during the granites of different ages. 



With these brief suggestions I pass to the second division 

 of the subject before us. 



2. Minerals occupying cavities and seams in amygdaloidal trap 

 or basalt. — These minerals have been attributed to a variety 

 of sources, and even at the present time there are various opi- 

 nions respecting their origin. According to some writers, 

 they result from the process of segregation ; that is, a sepa- 

 ration of part of the material containing rock during its cool- 

 ing by the segregating powers of crystallization ; and in illus- 

 tration of the process we are pointed to the many segregations 

 of felspar, quartz and mica, in granite and other rocks, the 

 siliceous nodules in many sandstones, the pearlstones in tra- 

 chytes and obsidian. Others have thought them foreign 

 pebbles, enclosed at the time the rock was formed. Again, 

 they are described as proceeding from the vapours which 

 permeated the rock while still liquid, and which condensed as 

 the rock cooled, in cavities produced by the vapours. By a 

 few it is urged, admitting that the cavities are inflations by 

 vapours like those of common lava, that they may have been 

 filled either at the time the rock cooled or at some subsequent 

 time, either by crystallization from vapours, or from infiltra- 

 ting fluids, but more generally the latter. 



Of these views we believe the last to accord best with the 

 facts. Macculloch, in his System of Geology — a work which 

 anticipated many of the geological principles that have since 

 become popular — dwells at length on this subject, and sup- 

 ports the opinion here adopted with various facts and argu- 

 ments. Lyell also admits the same principles. A review of 

 the facts will enable us to judge of its correctness. 



1. In the first place, the cavities occupied by the nodules 

 are in every respect similar to the common inflations or air- 

 bubbles in lava. These cavities are open and unoccupied in 

 common lava, and may be no less frequently so in the ejec- 

 tions under water; and should they not be expected to fill in 



