Licence: Public Domain Mark
Credit: On brecciated concretions / by John Ruskin. Source: Wellcome Collection.
Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.
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![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 silica in two different 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 fine 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 different 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. Tins 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 jasperiuo 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](https://iiif.wellcomecollection.org/image/b22395532_0012.jp2/full/800%2C/0/default.jpg)
