16 Buskin — On Brecciafed Concretions. 



then there is nearly always a dark rod in the stalactitic centre, 

 which may or may not be solid. In the finest Mocha stones, 

 I think it is a black film round a chalcedonic nucleus ; but in the 

 associations of limonite Tvith chalcedony, it is usually of solid 

 radiate iron-oxide, and doubtless of prior, though perhaps only of 

 immediately prior, formation. A more complex state is presented 

 by such stalactites, when enveloped in a chalcedonic solid paste, to 

 which they do not communicate their own zoned structure. Ordi- 

 narily, the surrounding mass throws itself into zones parallel mth 

 those of the enclosed stalactite ; but, in some cases, it is of quite 

 adverse structure, perhaps laid level across the stalactitic fall. 



The conditions admitting the interfusion of this solid paste,^ are 

 strangely connected with those which cause chalcedony to form true 

 vertical stalactites and straight rods, instead of arborescent and 

 twisted stalactites. I have never seen the twisted stalactite unless en- 

 veloping fibres of some foreign, perhaps organic, substance, enclosed 

 in massive chalcedony ; but the straight stalactite is perhaps oftener 

 so than free (unless connected with limonite), and it would appear, 

 therefore, as if the apparently interposed mass were really of contem- 

 porary formation, or else it would sometimes enclose the contorted 

 stalactite. But this question respecting the causes of the vertical 

 and twisted groups properly belongs to the second branch of our 

 inquiry as to states of repose. 



Second : Conditions afiecting mode of crystallization. It is evident 

 that fluent deposits of silica contained in a rock-cavity must be 

 affected, in course of their solidification, not only by every addition 

 to their own mass, but by every change in the temperature or grain 

 of the surrounding rock, so thai we have innumerable modifications 

 of state, dependent partly on accession and transmission of sub- 

 stance, partly on changes in external temperature and pressure. 

 And, under these influences, we perceive that the gelatinous silica 

 occasionally obeys gravity,^ and occasionally resists it, becoming 

 sometimes pendent from the roof, and forming level lakes on the 

 floor of cavities ; at other times, throwing parallel bands on floor and 

 roof alike, and in transitional periods, forming thick layers on the 

 floor, and thin ones at the sides, the layers being liable, meantime, 

 to different degrees of compression from their own modes of solidi- 

 fication, which give them, locally, the appearance of an elastic 

 compression and expansion : there seems no limit to the fineness of 

 their lines at these compressed points, when their continuity is unin- 

 terrupted. Figures 6 and 7 illustrate, in two small pieces of agate, 

 each here magnified about three times, most of the appearances 

 which must be severally studied. In Fig. 6 the lowest band, A. 

 level at the bottom, broken irregularly towards the rough side of 

 the stone, is yet of nearly even thickness everywhere ; above it, the 



1 I use this word gravity in some doubt ; not being quite sure that the straight 

 beds are always horizontal, or always inferior to the rest deposited at the same time. 

 I have one specimen in which, according to all analogies of structure, it wtmld appear 

 that the vacant space is under the level floor, between it and reniform chalcedony ; 

 and sometimes these floors cross pillars of stalactite like tiers of scaffolding. 



