from the Standpoint of Physical Chemistry. 223 



be perceived. Instead there is a beautiful development of 

 tufts and sheaves of feathery raicrolites occupying the entire 

 groundmass. The phenocrysts, as before, show groups of 

 diopside and plagioclase in mutual intergrowth. The general 

 dark brown color of the groundmass appears to be due to dust 



Fig. 3. Fig. 4. 



Fig. 3. Microlitic additions to phenocrysts of diopside and plagioclase. 

 (Slide No. 132.) Diameter of field l-5 mm . 



Fig. 4. Intermediate texture between microlitic and holocrystalline. 

 (Slide No. 58.) Diameter of field l'5 mm . 



and. grains of magnetite developed along the lines of growth 

 of the feathery microlites and outlining their form. 



In slides No. 96 and No. 58 (fig. 4) a step in advance is 

 shown toward the holocrystalline texture of a normal basalt 

 resulting from moderately slow cooling. The sharply defined 

 outline of the phenocrysts has been lost and their borders 

 fray out into the finely crystalline groundmass. The ground- 

 mass itself is coarser than in the earlier described sections 

 and the subhedral crystals are comparable in size to the 

 phenocrysts. Some of the magnetite is in distinct grains, but 

 much of it is still in the form of a dark dust lying between 

 the lighter minerals. 



Xos. 98, 99, and 18 (Photograph, fig. 5) show successively 

 coarser phases of crystallization, and Nos. 23, 24, and 25 show 

 the normally developed texture of these basalts. Nos. 47 and 

 64 represent the maximum of coarseness of crystallization 

 attained. 



In the normally crystallized basalt the essential constituents 

 are plagioclase, pyroxene (diopside), and magnetite, the first 

 two in crystalline forms, the last in dust, granules, or trellis- 

 like groups. (See fig. 6.) 



The j)lagioclase is in lath-shaped crystals, euhedral to subhe- 

 dral, generally showing two or three stripes of albite-twinning 

 lamellae. Pericline twinning is rarely present. Its average 



