12 Ruskin — On Brccciated Concretions, 



TV. — On Brecciated Concretions. 

 By John Ruskin, Esq., F.G.S. 

 (Continued from the November Number, p. 483.) 

 (PLATE III.) 



THE states of semi-crystalline silica are so various, and so con- 

 nected in their variety, that the best recent authorities have 

 been content to group them all with quartz, giving to each only a 

 few words of special notice ; even the important chapters of Bischof 

 describe rather their states of decomposition and transition than the 

 minerals themselves. Nevertheless, as central types, five conditions 

 of silica are definable, structurally, if not chemically, distinct ; and 

 forming true species : and in entering on any detailed examination 

 of agatescent arrangements, it is quite necessary to define with pre- 

 cision these typical substances, and their relation to crystalline quartz. 

 I. Jasper. — Opaque, with dull earthy fracture ; and hard enough 

 to take a perfect polish. When the fracture is conchoidal the mineral 

 is not jasper, but stained flint. The transitional states are confused 

 in fracture ; but true jasper is absolutely separated from flint by two 

 structural characters ; on a small scale it is capable of the most 

 delicate pisolitic arrangement ; and on a larger scale is continually 

 found in flame-like concretions, beautifully involved and contorted. 

 But flint is never pisolitic, and, in any fine manner, never coiled ; nor 

 do either of these structures take place in any transitional specimen, 

 until the conchoidal fracture of the flint has given place to the dull 

 earthy one of jasper; nor is even jasper itself pisolitic on the fracture, 

 being too close-grained. The green base of heliotrope, with a per- 

 fectly even fracture, may be often seen, where it is speckled with 

 white, to be arranged in exquisitely sharp and minute spherical con- 

 cretions, cemented by a white paste, of which portions sometimes 

 take a completely brecciated aspect, each 

 ; fragment being outlined by concave seg- 

 ments of circles (Fig. 1). Jasper is emi- 

 nently retractile, like the clay in septaria, 

 and in agates often breaks into warped 

 fragments, dragging the rest of the stone 

 into distortion. In general, the imbedded 



Fig. 1. fragments in any brecciated agate will be 



mainly of jasper ; the cement, clialcedonic, or quartzose. 



II. Flint. — Amorphous silica, translucent on the edges, with fine 

 conchoidal fracture. Opaque only when altered, nascent, or stained. 

 Never coiled, never pisolitic, never reniform ; these essentially ne- 

 gative characters belonging to it as being usually formed by a slow 

 accumulative secretion, and afterwards remaining unmodified (pre- 

 serving therefore casts of organic forms with great precision). It is 

 less retractile than jasper ; its brecciate conditions being not so much 

 produced by contraction or secession, as by true secretion, even when 

 most irregular in shape (as a row of flints in chalk differ from the 

 limestone fragments represented in Vol. IV. Plate XX. Fig. 3, which 

 might stand for a jasperine structure also). But there are innumer- 

 able transitions between these two states, affected also by external 



