280 A. C. LANE — GEOLOGIC ACTIVITY OF THE EARTH's GASES. 



Messrs Cross and Emmons confirmed from their wide experience the 

 lack of noteworthy caustic phenomena in the contacts of rhyolites, but 

 remarl^ed that signs of caustic action are also often absent in basaltic 

 contacts. They would not be expected to occur around partly cooled 

 basalts. 



Mr Cross also commented on the fact of observation that similar mag- 

 mas, in the same area, were in some cases accompanied by, and in others 

 free from, mineralizing agents. Thus the great laccolitic masses of por- 

 phyrite in the West Elk mountains, Colorado, were not accompanied 

 by contact phenomena indicating the presence of gaseous mineralizers, 

 while the diorite masses of the adjacent Elk mountains were surrounded 

 by broad contact-zones, in which the development of scapolite, vesuvi- 

 anite, garnet, pyroxene, specular hematite, and various other minerals 

 attested to the presence of chlorine, fluorine, and perhaps other agents 

 in the magma at eruption. 



Professor Crosby's paper, " Origin of the Coarsely Crystalline Vein 

 Granites on Pegmatites," which was also read at this meeting, in part 

 overlaps mine in a confirmatory way. 



