Buskin — On Brecciated Concretions. ] 3 



violence, which we shall have to examine carefully. Within these 

 nodular concretions, flint is capable of a subsequently banded, though 

 not pisolitic arrangement. (See Dr. S. P. Woodward's paper on banded 

 flints, in this Magazine, Vol. I. for October, 1864, p. 145.) 



III. Chalcedony. — Reniform silica, translucent when pure, opaque 

 only when stained, nascent, or passing into quartz. The essential 

 characteristic of chalcedony is its reniform structure, which in the 

 pure mineral is as definite as in wavellite or haematite, though when 

 it is rapidly cooled or congealed from its nascent state of fluent jelly 

 it may remain as a mere amorphous coating of other substances ; very 

 rarely, however, without some slight evidence of its own reniform 

 crystallization. The study of its different degrees of congelation in 

 agates is of extreme intricacy. As a free mineral in open cavities it 

 is actively stalactitic, not merely pendant or accumulative, but ani- 

 mated by a kind of crystalline spinal energy, which gives to its pro- 

 cesses something of the arbitrary arrangement of real crystals, mo- 

 dified always by cohesion, gravity, and (presumably) by fluid and 

 gaseous currents. 



There is no transition between chalcedony and flint. They may 

 be intimately mixed at their edges, but the limit is definite. Im- 

 pure brown and amber-coloured chalcedonies, and those charged 

 with great quantities of foreign matter, may closely resemble flint, 

 but the two substances are entirely distinct. Between jasper and 

 chalcedony the separation is still more definite in mass, jasper being 

 never reniform, and differing greatly in fracture ; but the flame-like 

 or spotted crin? ^on stains of chalcedony often approach conditions of 

 jasper ; and there is, I suppose, no pisolitic formation of any sub- 

 stance without jme inherent radiation, which associates it with 

 reniform groups, so that pisolitic jasper must be considered as partly 

 transitive to chalcedony. On the other hand chalcedony seems to 

 pass into common crystalline quartz through milky stellate quartz, 

 associated in Auvergne with guttate and hemispherical forms. 



IV. Opal. — Amorphous translucent silica, with resinous fracture, 

 and essential water. Distinguished from chalcedony by three great 

 structural characte >tics ; a, its resinous fracture ; 6, that it is never 

 pisolitic or reniform; c, that when zoned, in cavities or veins, its 

 zones are always rectilinear, and transverse to the vein, while those 

 of chalcedony are usually undulating, and parallel to the sides of the 

 vein ; level only in lakes at the bottom of cavities. 



V. Hyalite. — Amorphous transparent silica, 

 with vitreous fracture, and essential water. 

 Never reniform, nor pisolitic, nor banded ; but 

 composed of irregularly grouped bosses, gene- 

 rally elliptical or pear-shaped (only accidentally 

 spherical), formed apparently by successive ac- 

 cretion of coats, but not showing banded struc- 

 ture internally (Fig. 2). Entirely transparent, 

 with splendid smooth glassy fracture. Some- ^ ' 



times coating lava ; sometimes in irregularly isolated patches upon 

 it : apparently connected in structure with the roseate clusters of 



