50 Formation of Hyalite hy the Action of Ammonia. 



opaque or white clayey cement to a vitreous or quartz-like 

 material, eventually, as may be seen on the surface of many 

 of our sandstone ranges, to a dense quartzite. 



On the Grampian range, at the Blue range at Mansfield, 

 and at Freestone Creek in Gippsland, the rocks are usually 

 very hard silicious sandstones at the surface, and give when 

 crushed and washed little or no clayey matter ; but a few 

 inches, or at most a, few feet_, from the surface on the same 

 beds the character changes, and on crushing and washing 

 the cementing material may be obtained as a nearly white 

 clayey material like kaolin. 



I have to a limited extent succeeded in changing clayey 

 sandstones to hard silicious sandstones by causing them to 

 absorb ammonial solutions in such a manner that the liquid 

 was absorbed at one end of the stone and evaporated at 

 the other, and obtained an outer surface hard and sili- 

 cious like that found in nature. 



With stones containing silicia in a hydrous form, like the 

 Oamaru, New Zealand, limestone, the passage is most marked. 

 In a few weeks the outer or evaporating surface gave upon 

 analysis twice as much silica as the interior of the stone. 



Thus, besides mere transfer of silica, the ammoniacal 

 solutions of silica are capable of producing actual meta- 

 morphism, changing the character and structure of the 

 silicate rocks. 



Some eighteen months ago I placed some clean infusorial 

 earth from Talbot in a solution of ammonia. The whole of the 

 earth was composed of the transparent forms of diatoms, 

 i.e., nearly pure hydrous silica. Recently examining the 

 contents of the bottle, I find that a portion of the silica has 

 been dissolved in the ammonia, giving a solution containing 

 •771 per cent, of silica ; at 212 it lost 01 per cent., and 0-01 

 on heating to about 350. The amount of hydrous silica in 

 solution is therefore over 500 grains to the gallon, far in 

 excess of that held in solution in the waters of the hot 

 springs of New Zealand. 



The solution of silicate of ammonia may be boiled till all 

 excess of ammonia has been expelled, and according to 

 Pribram (Watts' Sup.), 1 equivalent of ammonia is left in 

 solution with 80 of silica. 



This boiled solution, in contact with bases, forms crystal- 

 lisable hydrous silicates. When evaporated to dryness it 

 deposits the silica as a film, which shrinks and cracks as the 

 last of the water is driven oflf. 



