198 Mr J. D. Dana on the Origin of Trap Minerals. 



includes lime instead of the alkalies, replaces common feld- 

 spar. The original feldspar seems to have given way to leu- 

 cite and Labradorite.* 



An important source of new combination is found in the 

 sea-water which gains access to the fires of volcanoes. The 

 decomposition which takes place eliminates muriatic acid, so 

 often detected among volcanic vapour ; but the soda and other 

 fixed constituents remain, to enter into combination with 

 some of the ingredients in fusion. Is not this one source of 

 the soda forming the soda feldspar, or albite, and of the mu- 

 riatic acid and soda in sodalite ? Phosphates have been long 

 known to occur occasionally in volcanic rocks, and lately phos- 

 phoric acid has been proved to be generally common in small 

 quantities. Sea-water is also a very probable source of this 

 ingredient, as has been shewn by late analyses of the same by 

 Dr Jackson. 



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 peculiarity 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 differ- 

 ent 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 cavilies 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 opinions respecting their origin. According to some 



* Using H for the bases and Si for silica, the formula of leucite is H Si^ ; 

 that of common feldspar, R Si ** ; that of Labradorite, R Si. From this, it ap- 

 pears that feldspar may be reduced to leucite by giving up one third of its 

 silica, the bases being the same in the two ; and with this excess, and other si- 

 lica combining with the lime at hand, Labradorite might be formed. 



