FENNER, THE WATCHVNG BASALT 137 



plentiful. There are many fracture-lines, into some of which a reddish- 

 brown deposit of sand and clay has been carried from the underlying 

 lake beds. Most of the slide is in the stage of recrystallization in which 

 zeolites are prominent, but in places has advanced to the stage of calcite 

 and kaolin. Stilbite and analcite can be recognized, and others may be 

 present. In the illustration, the radiating groups somewhat simulating 

 forms of vegetation are probably crystals of quartz surviving from a much 

 earlier period. They appear to branch from minute cracks. In some por- 

 tions of the slide they show strongly the effects of solution. As regards 

 the replacing mineral, it is not possible to be more specific than to say 

 that it is probably a zeolite. In 125, quartz crystals which have begun 



Fig. 13. Replacement of quartz by calcite. The quartz has the fan structure 

 generally supposed to be typical of vein quartz, but it has probably replaced 

 basalt. The granular-looking calcite sho^YS features which are plainly a 

 survival of those in the quartz. X 35. Slide 5S. 



their growth in similar groups radiating from cracks have reached a 

 large size. 



In slide 58, a dense, normally crystalline basalt is crusted with quartz 

 and chalcedony. The quartz has the fanlike crystallization supposed to 

 be typical of vein quartz, although certain circular lines and markings 

 (similar to those shown in fig. 2, though not so pronounced) suggest that 

 much of the quartz may have replaced a glassy crust of basalt. The 

 chalcedonic SiOj has a fibrous appearance and negative elongation. 

 Within the basalt, the normal texture is preserved, but with crossed nicols 

 it is seen that in places both diopside and plagioclase have been replaced 

 by a mosaic of quartz. In the outer crusts, calcite in a finely granular 

 form is working in and replacing the quartz. It is developed most 



