532 J. Buskin — On Banded and Brecciated Agates. 



of chalcedony were formed by superfluent coats, some of these coats 

 would have iron in solution at the outside as well as the interior, 

 and would secrete it in successive films ; whereas, on the contrary, 

 the entire bulk of the iron, being always central, must surely have 

 been secreted out of the entire mass ; and, therefore, I believe that 

 the true chalcedonic stalactite is indeed a long botryoidal crystal, 

 like some of the forms of sulphide of iron, found in chalk, and not 

 at all a drooping succession of fluent coats, except in cases of rapid 

 deposit, which, as far as I remember, show no central iron. 



Again, when iron is systematically associated with quartz, it is 

 never in the centre of the crystal, but either on the surface or under 

 an externally imposed glaze. Hence it follows that the crystalline 

 forces at work in forming quartz act nearly in the reverse of those 

 that form chalcedony, as regards the direction of ferruginous ele- 

 ments, and that they have quite a peculiar power in finishing crystals, 

 which determines, at a given time, either a purer, or an amethystine, 

 silica to the surface, often throwing down crystals of iron between 

 the two. 



I have already noticed the clear coat forming the exterior of many 

 nested agates in basaltic cells, and the deposit of iron succeeding it, 

 to which I gave the name of medial oxide. My impression is that 

 the exterior of such agates, as relating to the crystalline power, may 

 be considered identical with the centre of a stalactite, and I think it 

 will be found that the iron in such stalactitic centres, however deli- 

 cate the fibre of it, is not solid, but tubular, leaving the absolute 

 centre of clear silica correspondent to the surface of clear silica in 

 a quartz crystal. 



It is very strange that among these complicated forces certain 

 conditions of chalcedony and quartz should be so constant, and the 

 intermediate states, giving evidence of formation, so rare ; but 

 though the interior of almost every quartz crystal shows the forces 

 of agatescence and straight crystallization in confused contest, I have 

 only seven or eight specimens, out of a collection of some thousands, 

 •which clearly show the balance of the two powers in accomplished 

 structure. 



The uppermost figure in Plate XIX. represents a portion of one of 

 these, which is a stellar agate, formed of grey chalcedony, with white 

 bands collected in a knot within radiant quartz. The precision of its 

 lines is beyond all imitation, but Mr. Allen has succeeded in draw- 

 ing and engraving it for us quite well enough to show the repeated 

 efforts of the chalcedony to throw itself into straight crystalline planes, 

 successful, tremulously, here and there for a quarter of an inch, and 

 then thrust again into curvature by the lateral spheric force. 



The second example, engraved in the lower figure in Plate XIX., 

 shows the two forces reconciled in their reign : the ci'yslalline or 

 mural form is completely taken by the agatescent bands in one part 

 of the stone and the spheric in another, while the bands themselves 

 are arranged in double folds, turned at the extremities, like the back 

 of a book. 



Finally, the wood-cutj Figure 4, gives the rude outline of a stone 



