Rushin — On Brecc'iated Concretions, 



17 



one with a black central line encompasses the whole agate symmetri- 

 cally. Then a white band, thin at the bottom., projects into concre- 

 tions on the flanks. Then, a thick white deposit, B. does not ascend 

 at the flank at all ; then a crystalline bed, with pisolitic concretions 

 at the bottom of it, changes into dark chalcedony (drawn as black), 

 which ascends at the flanks. Then another thin line at the bottom, 

 in concretion at flanks ; then one thick at the bottom, thin at the 

 flanks, and so upwards, In Fig. 7, a level mass, itself composed of 



Fig. 6. Fig. 7. 



silica in two difi*erent states, one separating into flakes, and the other 

 even-laid, is surrounded by bands which melt into it with gradually 

 diminishing thickness, these being evidently subordinate to an ex- 

 ternal formation of crystalline quartz ; the whole terminated by a 

 series of flne bands of graduated thickness, and by clear chalcedony 

 (drawn as black). 



Now all these, and many more such variations, take place 

 without any apparent disturbance of the general mass, each bed 

 conforming itself perfectly to the caprice of its neighbour, and 

 leaving no rents nor flaws. But an entirely diflerent series of phe- 

 nomena arise out of the fracture or distortion of one deposit by 

 another, after the first has attained consistence. Thus, in Fig. 4, a 

 yellow orbicular jasper is split into segments, singularly stellate, or 

 wheel-like, and then variously lifted and torn by superimposed chal- 

 cedony ; and in Fig. 5, a white and opaque agatescent mass is rent, 

 while still ductile, the rents being filled with pure chalcedony : and 

 from this state, in which the pieces are hardly separate, and almost 

 hang together by connecting threads, we may pass on through every 

 phase of dislocation to perfect breccia ; but, all the while, we shall 

 find the aspect of each formation modified by another kind of fault, 

 which has no violent origin, and for the illustration of which I have 

 prepared Plate III. This plate represents (all the figures being of 

 the natural size) three sections of amethystine agate, in which the 

 principal material is amethyst-quartz, and the white jasperine bands 

 for the most part form between the points of the crystals. 



All the three examples are types of pure concrete agatescence in 

 repose, showing no trace whatever of external disturbance. The 

 fault in the inclined bed at the base of the uppermost figure, has 

 some appearance of having been caused by a shock ; but for that 

 reason is all the more remarkable, the bed beneath it being wholly 



VOL. V. — NO. XLIII. 2 



