158 Buskin — O71 Banded and Brecciated Concretions, 



while the nested agates rim into level or irregularly continuous 

 bands, and choke their cavities with confused net- work of quartz. I 

 have difficulty in finding convenient names for these two families of 

 agate ; but merely for reference to them in these papers, I shall call 

 those formed in knots, which are often conspicuously radiant in the 

 lines of their ciystals, '• stellar" agates; and those evidently formed 

 in cavities, '' nested " agates. 



I believe that the stellar forms, when independent, will be found 

 most frequently under circumstances admitting the possibility of 

 slow concretion at comparative!}^ low temperatures, while the nested 

 or bomb-like structure belongs characteristically to volcanic forma- 

 tions, in which the cavities might be filled by comparatively violent 

 infusion, and their contents in many cases quickly cooled. Both condi- 

 tions, of course, sometimes agree in all their processes ; and we shall 

 be able finally to classify these processes of deposit under description 

 which will apply equally to the stellar and nested forms, marking 

 afterwards the points of exceptional difference. Thus, for instance, 

 the most frequent of all the forms of tranquil deposit, uninterrupted 

 by flowing additions of material, is that in which a clear band of 

 chalcedony, perfectly equal in breadth throughout, is first formed 

 round the point (or branch) of nucleus, in stellar, or on the outer 

 wall of the cavity in nested, agate. But after this has been formed 

 in stellar agate, the succeeding belts will not usually show a minor 

 pisolitic structure, whereas, in nested agates, marvellous groups of 

 pisolitic hemispherical arches often rise from the inner surface of the 

 clear external chalcedony, in section, like long bridges crossing a flat, 

 and modify the whole series of bands above them ; but, again, with 

 this most important distinction between these and the bands of stellar 

 agate, — that stellar bands, the farther they retire from the nucleus, 

 usually throw themselves with increasing precision into circular 

 curves, till they sometimes terminate in perfect and exquisitely 

 drawn segments of spheres; while in nested agate, the bands, if 

 parallel, efface more and more the original minor curves as they 

 approach the centre of the nest, and sweep over them in broad in- 

 determinate lines, as successive coats of paint of equal thickness 

 efface the projections and roughnesses of the surface they cover, or 

 as successive falls of snow, undrifted, efface irregularities of ground. 

 And now, observe, we shall want a word expressive of an inter- 

 mediate condition between the states above defined as pisolitic and 

 reniform. A pisolitic mineral we define to be one which separates 

 into more or less spherical layers by contraction ; and this kind of 

 division takes place sometimes quite irrespectively of the crystalline 

 structure, and on the grandest, as well as the most minute scale. 

 In one of my specimens of Indian Sard, there are multitudinous 

 pisolitic flaws, exquisitely perfect in spherical curvature, dividing 

 the parallel bands of the agate transversely in every direction, look- 

 ing like little palea^ of chaff in its clear substance ; on a large scale, 

 the aiguilles of Chamouni are pisolitic, rending themselves into 

 curved layers five or six hundred feet in the sweep of their arcs, 

 variously crossing their cleavage (which is rectilinear), and often 



