Ruskin — On Brecciated Concretions. 



15 



curves ; while chalcedony will energetically so terminate itself ex- 

 ternally, but will, in ordinary cases, only develope its pisolitic struc- 

 ture subordinately, by forming parallel bands round any rough 

 surface it has to cover, without collecting into spheres, unless either 

 provoked to do so by the introduction of a foreign substance, or 

 encouraged to do so by accidentally favourable conditions of repose. 

 And here branch out for us two questions, both most intricate ; first, 

 as to the introduction of foreign bodies ; secondly, as to the crystal- 

 line disposition of chalcedony, under variable permission of repose. 



First — As to foreign substances. I assume that in true pisolitic 

 concretion, such as that of the jasper, roughly sketched in Fig. 3. 



Fig. 3. Fig. 4. V'lg. 5. 



(it is not a coral — the radiant lines are merely conventional indica- 

 tions of the grain of the jasper, so far as it is visible with a lens^), 

 no foreign body has provoked the orbicular arrangement. The jasper is 

 red ; the little dark circles are wells of pure chalcedony, each con- 

 taining within it a white ball of crystallized quartz, forming a star 

 on the section. The whole is magnified about three times in the 

 drawing, being a portion of a horizontal layer, alternating with solid 

 white jasper. It seems that the pisolitic structure is here truly 

 native ; but we must nevertheless grant the possibility that the balls 

 of quartz may have had some organic atom for their nucleus. On 

 the other hand, in the ordinary conditions of dendritic agate, in 

 which stalactites of chalcedony surround branches of clearly visible 

 chlorite,^ or of oxide of iron or manganese, I assume that in the 

 plurality of cases, such sustaining substances have been first de- 

 veloped, and the chalcedonic stalactite afterwards superimposed, 

 being, in the most literal sense of the word, *' superfluous " silica; 

 but I, nevertheless, see great reason for thinking that, in many cases, 

 the core of the group is only a determination to its centre of 

 elements which had been dispersed through the mass. In the 

 generality of Mocha-stones, the dendritic oxides, so far from being 

 an original framework, are clearly of subsequent introduction, radi- 

 cally following the course of fissures from which they float par- 

 tially into the body of the imperfectly congealed gelatinous mass ; 

 in other more rare, and singularly beautiful cases, the metallic 

 oxides ramify in curves in the intervals of the pisolitic belts, and 



^ In my woodcut diagrams I shall employ no fine execution ; they will be merely 

 illustrative, not imitative,— diagrams, not drawings. In the plates, on the contrary, 

 with Mr. Allen's good help, I shall do the best I can. 



2 Or green earth ? I cannot find any good account of the green substance which 

 plays so important a part in the exterior coats of agates, and Iceland chalcedonies. 



