42 



PEOCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 66. 



The minerals of these cavities have been referred to as : First gener- 

 ation, those which are the constituents of the surrounding rock and 

 seem to have only accidentally abutted agginst the cavity, namely, 

 albite, quartz, diopside, and titanite; and second generation, which are 

 considered to have been deposited on the others by hydrothermal 

 solutions, namely, epidote, fibrous hornblende, chalcopyrite, and chlo- 

 rite. In one thin section, however, typical fibrous hornblende was 

 seen incorporated in an albite crystal which was itself a later prod- 

 uct than the albite of the adjacent rock. 



The cavities vary from very minute or microscopic to some which 

 are 3 cm. across. The minerals in them may be described individu- 

 ally as follows: 



QUARTZ. 



The quartz is in the form of transparent slightly smoky crystals 

 interspersed with the albite crystals. These reach a length of 2 mm. 

 and are rather rich in faces. One such which was measured had the 

 development shown in figure 2 and gave the following forms and 

 angles : 



Forms and angles of quartz fron a rniarolitic cavity. 



In their development some of the forms do not agree with the 

 ymmetry of the class, a discrepancy doubtless due to twinning. 



The albite, which is the most abundant mineral in the cavities, 

 occurs in crystals which reach an extreme diameter of about 4 milli- 

 meters. They vary from opaque to transparent and from white to 

 pale pinkish. They all have the relatively simple form shown in 

 figure 3. One which was measured on the 2-circle goniometer gave 

 the following angles : 



