LITHOIDAL OR HOLYOKEITE BASE 107 



fibrous forms, showing abundant black crosses and the purity and bright 

 polarization in whites and grays which characterize water-deposited 

 quartz. The fibers are always positive, so that quartz and albite are 

 hard to distinguish. It is generally full of small clots or beads of glass, 

 forming an exceedingly fine brown dust. Sometimes the minute straight 

 plagioclase microlites show a fluidal arrangement; sometimes many 

 large and long crystals of apatite * and large square prisms of plagioclase 

 occur quite abundantly, the latter with a central thread of glass like a 

 minute chiastolite (see plate 30, figures 3 and 4). There are often many 

 small irregular patches of calcite grains. It seems to be only when the 

 glass is especially abundant that the aphanitic halo which surrounds it 

 shows an excess of silica by the blue color and effervescence with soda. 

 Generally there is only excess of silica enough to produce the micropeg- 

 matitic structure. When such a bluish piece of the lithoidal rock is 

 boiled for several hours in caustic soda a very considerable portion is 

 dissolved, and the few feldspars stand out in relief, and an unexpectedly 

 large number of minute pyrite cubes appear. The glass is. of course, 

 also deeply dissolved and the remnant is cracked and brown. When a 

 fragment is fused with soda it effervesces abundantly and is mostly 

 dissolved. 



THE HOLYOKEITE OR DIABASE-APLITE DIKES 



We pass by an easy transition from the isolated halos of the quartz- 

 albite mixture which surround the glass and forms the groundmass of 

 the coarsest gabbros described above to the small tertiary dikes of an 

 aphanitic trap of light color which prove to be identical in character 

 with the above groundmass (see plate 30, figures 3 and 4) and approach 

 holyokeite so closely in composition that it will be convenient to apply 

 the same name to both. They cut both the normal diabase and the 

 secondary coarse schlieren of glass-bearing diabase and in many cases 

 run down the middle of the coarse schlieren for long distances (see plate 

 25, figure 2). 



Under the microscope the dike material shows so many affinities to 

 the glass-bearing quartz-diabase schlieren that the idea that they are 

 more acid differentiates from the latter is strongly suggested. There is 

 the same abundance of yellow glass, here always devitrified, into which 

 long stout needles of plagioclase have penetrated, and the holyokeite 

 ground is identical in both (see plate 30, figure 4). Calcite in small 

 shapeless masses is often blended in the mass, and the fresh state of all 



* The apatites crystallizing in the albitic ground surrounding the glass would seem to add one 

 more case to the short list of exceptions to the rule that the apatite crystallizes first. The later 

 appearance is a proof of the rapid formation of the large poikilitic feldspars. 



XV— Bull. Gbol. Soc. Am., Vol. 16. 1904 



