THE DESERT SANDSTONE. 333 



It is not necessary to add further details about the microscopic 

 character of grains of sand derived from the Desert Sandstone in 

 other different places, since they are all of the most uniform 

 character, showing few opaque particles, a fair proportion of 

 volcanic glass, very transparent quartz grains with foreign 

 inclusions principally of chrysolite, but often with small fragments 

 of augite partly converted into hornblende, various kinds of felspars 

 and gas-cavities. 



For comparison, I insert a few notes on volcanic sands and ashes. 



Ash from Krakatoa. — This ash, aggregated into small rounded 

 grains like sago, caused by moisture of some kind, possibly the 

 steam connected with the volcano. Particles when spread out 

 very finely divided and more than half quite transparent, the 

 included fragments of magnetite and other sph?erulites very 

 numerous ; also gas cavities ; some of the grains having the 

 appearance of glassy froth of minute bubbles, in others the cavities 

 forming rough parallel lines like the irregular grain of woody 

 fibre ; glassy fragments with the cavities drawn out into a long 

 ribboned structure with occasionally aggregated crystals of augite, 

 biotite, chrysolite, and labradorite. About half the fragments 

 do not polarize. 



Taal Volcano, Luzon, Philippines. — Most of the larger particles 

 opaque, quartz and felspar whitish and transparent, full of brown 

 and black sphaerulites of magnetite and other minerals; gas cavities 

 not so numerous as the included minerals, but all very much 

 mingled as if frothy. Polariscope — only a very few fragments 

 polarize belonging to the finest dust, the glassy particles however 

 are frequently opaque. 



Sand Sea, Bromo, Java. — More than half the particles opaque, 

 transparent particles polarizing brilliantly with few exceptions, 

 some of the fragments showing crystals of labradorite and other 

 felspars besides olivine and augite. Cavities and sphaerulites 

 numerous, making most of the transparent grains frothy looking. 

 Some of the opaque grains with crystalline faces. The actual 

 crater of Bromo is a hill composed of the finest possible dust which 

 is incessantly discharged with a loud series of explosions. These 

 follow each other so closely as to make a roaring noise, that can 

 be heard from a long distance. The dust thus discharged is a 

 brownish powder, about half the particles of which are glassy and 

 refuse to polarize. There are a few dark, opaque granules, the 

 rest being transparent with comparatively few included microliths. 

 Augite, olivine, and twin felspar crystals and microliths of 

 felspar very numerous. 



As a rule, it may be generally stated that the volcanic sands 

 are darker and contain a very much larger proportion of opaque 

 fragments than the grains of the Desert Sandstone. In polarizing 



