ART. 2. 



PETROLOGY AT GOOSE CREEK SHANNON. 



41 



clase by albite, augite by diopside, and magnetite by titanite. It is 

 consequently difficult to definitely separate magmatic from hydro- 

 thermal action. 



The process is clearly a special phase of crystallization differentia- 

 tion, which may be termed ''pegmatitic differentiation," and presents 

 certain peculiarities depending 

 upon the abundant presence of 

 volatile constituents in the magma 

 system. While there are certain 

 hiatuses in the normal sequence 

 which can not now be explained, 

 it is important that the processes 

 are essentially the same as those 

 outlined by Bowen in his discus- 

 sion based upon the investigation 

 of anhydrous systems. It is prob- 

 able that the differentiation of 

 any natural magma is speeded up 

 and that the reactions are facili- 

 tated by the presence of volatile 

 constituents. 



The behavior of the water re- 

 leased after crystaUization of the 

 final magmatic product is further 

 discussed below. 



MIAROLITIC CAVITIES. 



Under the term ''miarolitic cav- 

 ities" are comprised smaU open 

 spaces which occur in the coarser 

 albitic rocks and are lined with 

 quartz and albite crystals. In most 

 cases in the rocks rich in micropeg- 

 matite these cavities are at the 

 junction of several areas of micro- 

 pegmatite, and the albite and 

 quartz crystals on the walls are as a 

 rule continuous with the same min- 

 erals of the adjacent rock and have 

 only formed euhedral crystals be 

 cause of space being available for 

 development. Entirely similar cavities occur also in coarse albite 

 rocks considered as possibly having originated through hydrothermal 

 alteration of normal plagioclase-pegmatite. 

 94110—24 4 



Fig. 2 — Quartz— SHOWING highly modified 



DEVELOPMENT OF QUARTZ CRYSTALS OCCURRING 

 IN MIAROLITIC CAVITIES. 



