98 



HA RD WICKE 'S S CIENCE - G OS SI P. 



oxides of the metals from the colloid or partially 

 crystallized silica ; in other cases they may be the 

 effect of subsequent infiltrations ; or again, the quartz 

 may have been consolidated around previously existing 

 crystallizations. With regard to infiltration by these 

 oxides, it is well known that even the most compact- 

 looking chalcedony is permeable, as it is possible by 

 steeping it in solutions of the aniline or other dyes to 

 impart the most brilliant tints to agates, the dye un- 

 doubtedly gaining access to the interior ot the speci- 

 men through the interspaces of its minutely crystalline 

 structure. 



In a large gi'oup of agates, of which beautiful speci- 

 mens come from India, an appearance of banded 

 formation is seen, which, upon microscopic examina- 

 tion, resolves itself into an infinite number of red or 

 brown spots, regularly arranged in bands or concen- 

 tric groups : these spots appear to be segregations of 

 oxide of iron. I have not seen a specimen of this 

 species of agate cut sufficiently thin to show whether 

 the arrangement of these minute spots is dependent 

 upon a banded structure in the chalcedony itself, or 

 whether it is independent and the result of molecular 

 force which has determined the arrangement in ques- 

 tion. It may here be noticed that a vast number of 

 the Indian agates come from the neighbourhood of 

 the Gulf of Cambay. Near Turkeysar there are 

 agate conglomerates intercalated between beds of 

 laterite which belong to the Eocene period. These 

 conglomerates we may suppose to have been derived 

 from the denudation of the earlier igneous rocks 

 which abound in the same district. Uruguay, in 

 South America, also produces a large number of 

 remarkably fine banded agates. Sometimes well- 

 formed quartz crystals will be found inclosing 

 other substances, which, in some instances, have 

 been caught up by the crystals in the course of their 

 formation, or have crystallized, perhaps, almost 

 simultaneously with the quartz. In other cases the 

 quartz is proved to have crystallized over other pre- 

 viously-formed crystals ; thus schorl is occasionally 

 seen partially inclosed in quartz crystals and partially 

 free, the ends of the crystals of schorl projecting 

 through the quartz. Titanite, asbestos, and other 

 minerals are not unfrequently found in minute acicu- 

 lar forms in quartz. The quartz in the igneous rocks 

 may frequently be seen to inclose crystals of felspar 

 or titanite, or portions of the matrix which must 

 have been previously solidified. 



Opal, as has already been pointed out, is a pro- 

 duct of aqueous origin found in the fissures and 

 amygdaloid cavities of igneous rocks. Its wondrous 

 play of colours has given rise to much discussion by 

 Brewster, Des Cloiseaux, and other writers. Some 

 have attributed it to the presence of numerous -cavi- 

 ties of varying size, which cause a kind of iridescent 

 refraction. Des Cloiseaux was inclined to suppose 

 that organic matter might be inclosed in small 

 quantities in its cavities. The most reasonable sup- 



position, however, appears to me to be that of 

 Reusch, — that light reflected or transmitted from 

 numberless flaws in the mineral gives rise to the 

 phenomena in question through a process of double 

 refraction. 



We may now turn to the consideration of forms of 

 quartz which have a more or less organic origin. At 

 the head of these may be placed such undoubtedly 

 organic aggregations of silica as the Tripoli and 

 semi-opal of Bohemia, which consist almost entirely 

 of fossil diatomacece. Some beds of rock also in the 

 island of Barbadoes are found to be composed of 

 little else than polycystinje and spiculaa of sponges. 

 Much of the flint so characteristic of the chalk rocks, 

 as well as the chert of the greensand and of the 

 mountain limestone, appears to have been derived 

 from the precipitation, by organic substances, of 

 silica held in solution by the waters of the ocean ; at 

 any rate, much of it seems to have been thus de- 

 posited ; flinty nodules are often found to consist of 

 fossilized sponges, the silicious skeletons of which 

 may have attracted to themselves the silica dissolved 

 in the surrounding water. Spiculie of sponges, dia- 

 tomacea;, foraminifera, shells, corals, and other organ- 

 isms are abundant in the flint, and also in much of 

 the chert. Recent observations by MM. Guignet and 

 Teller have shown that the water of the Bay of Rio- 

 de Janeiro contains large quantities of both silica 

 and alumina in solution, the amount in the case of 

 silica being as much as 9.5 grains per cubic meter. 



Wood will sometimes be found to be pseudomor- 

 phosed into silica, the woody structure being replaced 

 atom by atom, so that the minutest vessels are per- 

 fectly preserved. Various species of palm from the 

 East Indies are frequently found fossilized in this 

 manner, and sections of them make very beautiful 

 objects for the microscope. Large fragments of a 

 partially silicified wood, named Endogcnitcs crosa, 

 may often be found in the neighbourhood of Hastings,. 

 j derived from the Wealden formation. 

 1 The curious so-called mineral Beekite is really 

 ■ coral or shelly matter which has been replaced by 

 silica. Researches into the behaviour of the colloid 

 fonn of silica, already spoken of, have shown how in 

 \ many instances large deposits of silica, such as the 

 '' flinty bands of the cretaceous formation, may have 

 originated. Mr. Church's experiments, made some 

 years since, proved that the minutest particle of car- 

 i bonate of lime was sufficient to transform the pure 

 aqueous solution of silica into the solid state in the 

 ! course of a few minutes ; and he was able, by the 

 I infiltration of silica in solution, to replace almost 

 ! entirely the carbonate of lime in recent coral by 

 1 silica, producing by this means what may be looked 

 : upon as a kind of artificial Beekite. Thus in the 

 slower, perhaps, but mighty chemistry of nature, 

 j marvellous re-actions may have taken place, giving 

 j rise to- some of the multitudinous forms in which 

 silica presents itself to the mineralogical student. 



