Rusldn — Banded and Brecciated Concretions, 



211 



bedded stones in which it seems to be of ordinal}^ occurrence) : and 

 also, the beds ought to present some of the irregularly accumulate 

 aspect of common calcareous stalactite; and in the interior we 

 ought to find sometimes vacancies left by the failure of supply. 

 But on the contrary, folded agates are always full, so far as I have 

 seen, except occasionally in the centres of their tubes, or in hollows 

 of outer folds, but they are always closed in their centres (differing, 

 observe, again essentially from common agate in this circumstance), 

 and their beds are not only parallel, instead of irregularly heaped, 

 but involved in the strangest way in reduplicate crystalline series. 

 See the interior of the stone, Fig. 2, in Plate XIII. 



On the other hand, were they truly concrete, these beds ought to 

 exhibit occasionally clear evidence of subordinate concretion in their 

 mass. Thus in the true concrete jasperine agate, Fig. 4/ the beds 



which are simply concurrent on the right hand break up presently, 

 and separate into flamy and shell-like groups, transverse to the 

 general bedding, and at last bend round a knotted nucleus ; but 

 nothing of this kind ever occurs in folded agates, though their veils 

 of dependent film are sometimes covered with an exquisite dew of 

 minute pisolitic concretions, making them look (under the lens) like 

 a beautiful tissue of gossamer laden with dew, and connected with a 

 peculiar complex basalt-like fracture : then finally, to finish the 

 difficulty, these folded agates are connected by a series of scarcely 

 distinguishable transitions with the group which we shall have next 

 to examine, which seems to be in great part concretionary, but 

 concretionary in right lines. The two lowest figures in Plate XIII. 

 are outlines of two of the most singular conditions of it. Fig. 3, 

 Plate XIII. is reduced in scale from a stone which I shall hereafter 

 engrave of its real size, as its mode of association of agatescent with 

 crystalline structure is, as far as I know, unique — and its proper 

 discussion is connected with that of the modes of increase of crystals. 

 Fig. 4, Plate XIII. is from an agate of almost equal rarity, though 

 I have seen other examples of its structure, but never so decisive in 

 character. This figure is slightly enlarged, being of a portion of a 

 mass which has crystallized out of a breccia, in thin walls of linear 

 brown agate enclosing opaque white agate, leaving internal spaces 

 filled with quartz. 



The entire group to which these examples belong, consisting of 



^ Magnified abmit tlirpe timpja. 



