VEINS OF QUARTZ, CALCITE, AND TUFF 111 



explosion may have reopened the fissure on the other side of c and per- 

 mitted its entrance. 



This seems to harmonize with the explanation elsewhere advanced 

 in this paper to explain the appearance of the holyokeite ground spo- 

 radically in the coarse diabase, namely, abnormal conditions stimulate 

 the overabundant growth of the pyroxene and magnetite, and the albitic 

 magma remains as a sort of caput mortuum and crystallizes where it 

 can find place either interstitially or squeezed out in small dikes. 



QUATERNARY VEINS OF QUARTZ, OF CALCITE, AND OF TUFF 



Small veins of quartz and calcite cut all the three types of diabase 

 described above. They contain at the border much pyrite and a little 

 calcite and are often distinctly bluish. 



Other similar veins are so loaded with the finest dust of the trap that 

 they may be called tuff veins. In the midst of the coarse trap occur 

 these apparent dikes of fine-grained trap, which prove to be made up of 

 fine dust of the coarse trap of the type in which much of the diabase has 

 been remelted. A slide was cut from a half-inch vein. This is bordered 

 on each side by a band of the radiate granular quartz. This band is 

 doubled on one side as if after the fissure had been injected full of the 

 fine mud produced by the friction of the walls of the trap on each other 

 the superheated waters had cemented the whole with quartz and then 

 the cavity had widened, so as to separate the solidified mud from the 

 walls (or the mud shrank on solidifying), and the narrow space on either 

 side had filled with the pure quartz, and the fissure widening again on 

 on one side a second comb of quartz had come in to fill the second cavity. 



The contents of this vein are curiously obliquely banded as if the 

 crack had had a dip of 20 degrees, and layers of coarse and of fine stuff 

 had been dusted in so as to lie horizontally. Others are full of finer 

 dust and grains of the granular quartz and calcite cement. There is 

 always a thin band of pure radiate quartz, separating each fragment of 

 trap from the common ground, and there are also minute veins of pure 

 dolomite or ankerite separating these tuff veins from the adjacent trap. 



The amount of the ferruginous carbonate is so great that the veins are 

 always brown on the surface. This allies them with the many coarser 

 veins in the central breccia zone. Pyrite in small cubes is common in 

 these veins, and in one case beautiful octahedra of sphalerite occur, which 

 are simple model-like twins after (111). 



