338 Buskin — Banded and Brecciated Concretions. 



5. Segregation accompanied by crystalline action, passing into 

 granitic and poirphyritic formations. 



Of these the fourth mode of change is one of peculiar and varied 

 interest. I have endeavoured to represent three distinct and pro- 

 gressive conditions of it in the plate annexed ; but before describing 

 these, let us observe the structure of a piece of common pisolite from 

 the Carlsbad Springs. 



It consists of a calcareous paste which arranges itself, as it dries, 

 in imperfect spheres, formed of concentric coats which separate 

 clearly from each other, exposing delicately smooth surfaces of con- 

 tact : this deposit being formed in layers, alternating with others 

 more or less amorphous. Now it is easy to put beside any specimen 

 of this pisolite, a parallel example of stratified jasper, in which some 

 of the beds arrange themselves in pisolitic concretions, while others 

 remain amorphous. And I believe it will be found that the bands of 

 agate, when most distinct and beautiful, are not successive coats, but 

 pisolitic concretions of amorphous silica. 



Of course, however, the two conditions must be often united. In 

 all minerals of chalcedonic or reniform structure, stalactitic additions 

 may be manifestly made at various periods to the original mass, 

 while in the substance of the whole accumulation, a structural 

 separation takes place, — separation (if the substance be siliceous) into 

 bands, spots, dendritic nuclei, and flame-like tracts of colour. But 

 the separation into any of these states is not so simple a matter as 

 might at first be supposed. 



On looking more closely at the Carlsbad pisolite, we may discern 

 here and there hemispherical concretions, of which the structure 

 seems not easily to be accounted for; — much less when it takes place 

 to the extent shown in Fig. 1, Plate XV., which represents, about 

 one-third magnified, a piece of concretionary ferruginous limestone, 

 in which I presume that the tendency of the iron-oxide to form reni- 

 form concretions has acted in aid of the pisolitic disposition of the 

 calcareous matter. But there is now introduced a feature of notable 

 difference. In common pisolite, the substance is homogeneous ; here, 

 every concretion is varied in substance from band to band, as in 

 agates; and more varied still in degree of crystalline or radiant 

 structure ; while also sharp-angled fragments, traversed in one case 

 by straight bands, are mingled among the spherical concretions : and 

 series of brown bands, of varying thickness, connect, on the upper 

 surfaces only, the irregular concretions together, in a manner not 

 unusual in marbles, but nevertheless (to me) inexplicable. 



Next to this specimen, let us take an example of what is usually 

 called "brecciated " malachite (Fig. 2, in the same plate). I think 

 very little attention will show, in ordinary specimens of banded 

 malachite, that the bands are concretionary, not successive ; and in 

 the specimen of which the section is represented in the plate, and in 

 all like it, I believe the apparently brecciated structure is concre- 

 tionary also. This brecciation, it will be observed, results from two 

 distinct processes : the rending asunder of the zoned concretions by 

 unequal contraction, which bends the zones into conditions like the 



