332 THE DESERT SANDSTONE. 



outburst on the earth's surface of very long ago, and perhaps the 

 great oriental fissure which extends from Sumatra to the Philippines 

 is its substitute. Had it not been for this liery epoch all Australia 

 would be utterly barren and fruitless, so we owe to its energy 

 something more than the stony arid plateaux of Desert Sandstone. 



Microscopic Appearance of Sands. — I now proceed to give the 

 result of microscopic examination of the sand grains showing how 

 far they afford evidence of their volcanic character. 



Sandstone from Mary River, North Australia. — Grains all 

 perfectly transparent, with no opaque particles, but in shape 

 singularly rounded ; all the rugged angles smoothed off; most of 

 the particles spherical and some completely egg-shaped. Foreign 

 inclusions not many ; sometimes stained by oxidation a brownish- 

 red (pseudomorphs after olivine?) more rarely grass-green (viridite?) 

 Cavities not numerous, but sometimes arranged thickly in lines of 

 trichites or rows of bubbles. Polariscope — about one-fifth of the 

 grains are dark, with crossed nicols. The rest polarize brilliantly, 

 rarely showing mineral inclusions of felspar. The number of glassy 

 particles in this sand with occasional crystals of hornblende and 

 other minerals, certainly suggest a volcanic origin. The rounded 

 outline is due more probably to melting than attrition, 



Fort Darwin Jail Road. — Grains angular, none rounded, mostly 

 transparent, but a few opaque, all more or less reddened by 

 oxidation. Foreign inclusions somewhat numerous, cavities very 

 numerous, sometimes taking the form of lines of trichites. 

 Polariscope — about a third glassy, refusing to polarize or only 

 feebly, the rest brilliantly, with the characteristic appearances of 

 chrysolite, augite and felspar crystals broken and fragmentary. 

 The large proportion of volcanic glass and included microliths give 

 it certainly a volcanic appearance. 



Fanny Bay, Beach Sand. — This sand is largely intermixed with 

 marine remains, notably foraminifera ( Quinqueloculina, Rotalia, 

 Planorbulina, Globigerina, Textularia,&c.) echini spines, fragments 

 of shells, corals, bryozoa, Gorgonia spicules and the usual tropical 

 marine exuviae, with opaque granules of magnetite, ferric oxides 

 and a few crystals of chrysolite, transparent grains of quartz with 

 included cavities and microliths. Polariscope — the few grains of 

 quartz polarize freely and show inclusions of felspar, augite, 

 chrysolite, and other volcanic minerals, all more or less changed. 

 Some perfectly formed crystals of chrysolite and few glassy 

 fragments. The matrix of volcanic minerals with marine exuvias 

 is very marked in this sand, but probably some of these may have 

 drifted from the active craters of the Moluccas and the islands to 

 the northward. The transparent sand-grains though few in 

 number are somewhat like those of the Mary River. 



