Buskin — Banded and Brecciaied Concretions. 339 



twisted fibres of a tree; and the filling up of the intervals with 

 angular fragments, mixed with an ochreous dust (represented in the 

 plate by the white ground), whUe the larger concretions of malachite 

 are abruptly terminated only at right angles to the course of their 

 zones, not broken raggedly across : a circumstance to be carefully 

 noted as forbidding the idea of ordinary accidental fracture. 



Whether concurrently with, or subsequent to, the brecciation (I 

 believe concurrently), various series of narrow bands have been 

 formed in some parts of the mass, binding the apparent fragments 

 together, and connecting themselves strangely with the unruptured 

 malachite, like the brown bands in example No. 1. 



Now, if we compare this condition of the ore of copper with such 

 a form of common brecciated agate as that represented in Plate XV., 

 Fig. 3, it will, I think, be manifest that the laws concerned in the 

 production of this last — though more subtle and decisive in opera- 

 tion, are essentially the same as those under which the malachite 

 breccia was formed, — complicated, however, by the energetically 

 crystalline power of the (amethystine) quartz, which exerts itself 

 concurrently with the force of segregation, and compels the zones 

 developed by the latter to follow, through a great part of their 

 course, the angular line of the extremities of the quartz crystals co- 

 temporaneously formed, while, in other parts of the stone, a brecciate 

 segregation, exactly similar to that of the malachite, and only the 

 fine ultimate perfectness of the condition of fragmentary separation 

 which is seen incipiently in the pisolite (Fig. 1), interrupts the con- 

 tinuity both of the agate and quartz. 



And finally, a narrow band, correspondent to the connecting zones 

 of the malachite, surrounds the brecciated fragments in many places, 

 while in others it loses itself in the general substance of the massive 

 quartz. 



I cannot, however, satisfy myself whether, in this last example, 

 some conditions of violent rupture do not mingle with those of 

 agatescent segregation ; and I am sincerely desirous to know the 

 opinions of better mineralogists than myself on these points of doubt : 

 and this the more, because in proceeding to real and unquestionable 

 states of brecciate rock, such as the fractured quartz and chalcedony 

 of Cornwall, I cannot discern the line of separation, or fix upon any 

 test by which a fragment truly broken and cemented by a siliceous 

 paste which has modified or partly dissolved its edges, may be dis- 

 tinguished from a secretion contemporaneous with the paste, like the 

 so frequent state of metalliferous ores dispersed in quartz. 



Hoping for some help therefore, I will not add anything further in 

 this paper; but if no one else will take up the subject, I shall pro- 

 ceed next month into some further particulars. 



EXPLANATION OF PLATE XV. 



Fig. 1. Section of a piece of concretionary ferruginous limestone, magnified about one- 

 third. 

 • Fig. 2. Section of a (so-called) " Brecciated " Malachite. 

 Fig. 3. Section of a Brecciated Agate. 



