14 Buskin — On Brecciated Concretions, 



milky chalcedony of Auvergne. I shall keep the term " guttate " 

 for this particular structure, of which singular varieties also occur 

 among the hornstones of Cornwall. 



These five main groups are thus definable without embarrassment : 

 two other conditions of silica, perhaps, ought to be separately named ; 

 namely, cacholony, which seems to take a place between chalcedony 

 and opal, but which I have not yet been able satisfactorily to define ; 

 the other, the calcareous-looking, usually whitish agate, which often 

 surrounds true translucent agate, as if derived from it by decom- 

 position. I am under the impression that this is chalcedony, more 

 or less charged with carbonate of lime, and that it might be arranged 

 separately as lime-jasper, differing from aluminous jasper by being 

 capable of reniform structure ; but it is certainly in some cases 

 an altered state of chalcedony, which seems in its more opaque zones 

 to get whiter by exposure to light. I shall therefore call it white 

 agate, when it harmoniously follows the translucent zones ; reserving 

 the term jasper for granular aggregations. Perhaps ultimately it 

 may be found that nascent chalcedony can take up either oxide 

 of iron, or alumina, or lime, and might relatively be called iron- 

 jasper, clay -jasper, and lime-jasper ; but for any present descriptive 

 purpose the simpler arrangement will suffice. 



These, then, being the principal types of agatescent silica, it is of 

 importance to define clearly the two structures I have severally 

 called pisolitic and reniform. 



A pisolitic mineral is one which has a tendency to separate bj'' 

 spherical fissures, or collect itself by spherical bands, round a 

 central point. 



A reniform mineral is one which crystallizes in radiation from 

 a central point, terminating all its crystals by an external spherical 

 surface. It is, however, difficult to define this character mathe- 

 matically. On the one hand, radiate crystals may be terminated by 

 spherical curves, as in many zeolites, without being close set enough 

 to constitute a reniform mass ; on the other, radiate crystals, set close, 

 may be terminated so as to prevent smoothness of external spherical 

 surface, and I am not sure whether this smoothness is a mere 

 character of minute scale (so that chalcedony, seen delicately enough, 

 might present pyramidal extremities of its fibres on the apparently 

 smooth surface), or whether, in true reniform structure, the crystal- 

 lization is actually arrested by a horizontal plane : I do not mean a 

 cystalline plane, as in beryl, but one of imperfect crystallization, 

 presenting itself only under a peculiar law of increase. Thus, in 

 haematite, which is both reniform and pisolitic, the masses often 

 divide in their interior by surfaces of jagged crystallization, while 

 externally they are smooth and even lustrous ; but I put this point 

 aside for future enquiry, because it will require us to go into the me- 

 thods of possible increment in quartz-crystals, and for our present pur- 

 pose, we need only a clear understanding of two plainly visible con- 

 ditions of jasper and chalcedony, namely, that jasper will collect itself 

 pisolitically, out of an amorphous mass, into concretion round central 

 points, but not actively terminate its external surface by spherical 



