PROCEEDINGS OE GEOLOGICAL SOCIETIES. 



67 



2'6, All quartz, for example, has this specific gravity, and not only quartz, 

 but chalcedony, hornstone, and flint, and yet these present no outward 

 sign of crj^stalline structure. It is, however, maintained by Rose, and 

 with some plausibility, that they consist of an aggregation of excessively 

 minute crystals. He designates these forms of quartz as crystalline, in 

 contradistinction to the ordinary form of rock crystal, which is distinctly 

 crystallized. We have, then, crystalline quartz, and this apparently non- 

 crystailized form of quartz just mentioned, chalcedony and the like, of the 

 high specific gravity 2*6. 



There is another form of silica in which the specific gravity never exceeds 

 2*3. It ranges from 2*2 to 2'3, and is never higher than that. This is 

 what is termed amorphous, apparently non-crystalline silica ; and these 

 facts have, or may have, a very important bearing on certain geological con- 

 siderations. All the crystallized silica of the high specific gravity polar- 

 izes light. The amorphous silica of low specific gravity does not polarize 

 light. The distinctly crystallized silica which we have in quartz, when 

 pulverized, reduced to extremely fine powder by trituration and levigation, 

 does not differ chemically in any sensible respect from the powder of the 

 apparently amorphous form of silica, flint, chalcedony, and the like. Both 

 resist the action of boiling alkaline solutions, whereas the amorphous silica 

 is copiously and readily dissolved by such solutions. The crystallized silica 

 is produced in the wet way, and, so far as we know, only in the wet way. 

 By the wet way is meant through the agency of liquids, never by fusion 

 at a high temperature. The late M. Senarmont, who devoted considerable 

 attention to the artificial formation of minerals, made microscopic c^stals 

 of quartz by dissolving silica in the nascent state in very dilute hydro- 

 chloric acid, and then exposing that solution in a closed tube to a tempera- 

 ture of between 200 and 300 centigrade ; exactly similar, in all essential 

 respects, to the rock-crystal of nature. It is true that the crystals were 

 very small ; but that in no way affects the truth of the conclusion. Sorby 

 obtained crystalline silica by passing chloride of silicon into a tube along 

 with the vapour of water, but he afterwards procured still more distinct 

 crystals by decomposing glass at a high temperature by the agency of 

 water. We may take an ordinary piece of glass and boil water in it for 

 almost any length of time, without appreciably acting upon it ; but if we 

 expose ordinary glass to the action of water at a high temperature in 

 a close vessel, the result is different, and the glass is rapidly attacked and 

 corroded. By acting upon glass consisting of silica, lime, and potash, by 

 water, at a high temperature, he obtained the well-known mineral Wollas- 

 tonite, which is a silicate of lime ; and he obtained perfectly transparent 

 crystals of quartz, not less than two millimetres in length, — a distinct ex- 

 perimental proof of the formation of characteristic crystals of quartz, 

 similar to those occurring in nature, by the agency of water. It is 



together, and hence we may infer the similarity of the conditions of their 

 formation. 



To avoid confusion about crystalline and amorphous silica, it must be 

 borne in mind that there is quartz which is distinctly, manifestly crystal- 

 lized with the specific gravity of 2*6. Then we have silica of the same 

 specific gravit} 7 ", yet not appearing crystalline to the eye, although there 

 are certain reasons for supposing that it may be composed of an aggre- 

 gation of excessively minute crystals. Then there is the other distinctly 

 amorphous non-crystalline variety, which has the low specific gravity of 

 2*3. Thus there are two apparently amorphous varieties of silica, — chal- 

 cedony, for example, and opal. If we compare common quartz with opal, 



a fact that the heavy compact 



occur 



