186 THE POPULAR SCIENCE MONTHLY. 



up by the crystals in the course of their formation, or have crystal- 

 lized, perhaps, almost simultaneously with the quartz. In other cases 

 the quartz is proved to have crystallized over other previously-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, asbestus, and other minerals are not un- 

 frequently found in minute acicular forms in quartz. The quartz in 

 the igneous rocks may frequently be seen to inclose crystals of feldspar 

 or titanite, or portions of the matrix which must have been previously 

 solidified. 



Opal, as has already been pointed out, is a product of aqueous origin 

 found in the fissures and amygdaloid cavities of igneous rocks. Its 

 wondrous play of colors has given rise to much discussion by Brewster, 

 Des Cloiseaux, and other writers. Some have attributed it to the 

 presence of numerous cavities 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 supposition, 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 diatoma- 

 cese. Some beds of rock also in the Island of Barbadoes are found 

 to be composed of little else than polycystince and spicules 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 deposited ; 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. 

 Spiculse of sponges, diatomaceae, foraminifera, shells, corals, and other 

 organisms 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 metre. 



Wood will sometimes be found to be pseudomorphosed into silica, 

 the woody structure being replaced atom by atom, so that the minutest 

 vessels are perfectly 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 frag- 

 ments of a partially silicified wood, named Endogenites erosa, may 



