PEOCEEDIXOS OF 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 crystalline structure. It is, however, maintained by Eose, and 
with some plausibility, that they consist of an aggregation of excessively 
minute crystals. He designates tliese 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- 
crystallized form of quartz just mentioned, chalcedony and the like, of the 
high specific gravitv 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 cr3^stallized silica of the high specific gravity polar- 
izes light. The amorphous siiica 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 tlie powder of the 
apparently amorphous form of silica, flint, chalcedonj', 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 M ay is meant through the agency of liquids, never by fusion 
at a higli temperature. The late M. Senarmont, who devoted considerable 
attention to the artificial formation of minerals, made microscopic crystals 
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- 
touite, 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 ])roof of the formation of characteristic crystals of quartz, 
similar to those occurring in nature, by the agency of water. It is 
a fact that the heavy compact and the crystallized silica often occur 
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 cr^'stal- 
lized with the specific gravity of 2'6. Then we have silica of the same 
specific gravitj^ 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 variet}^ 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, 
