210 RusJiin — Banded and Brecciated Concretions, 



that, taking Fig. 3 as a formal type of a perfect folded agate, tlie 



points a, b, c, etc., at the sides of the 

 nest have been those of impeded secre- 

 tion or deposit (if, which is not by any 

 means clear to me, there has been suc- 

 cessive deposit at all), and that the in- 

 termediate curved beds are the increas- 

 ing stalactitic masses. The right lines 

 indicating flaws at the intersection of 

 these masses, are essential in the typi- 

 cal structure. The two upper figures 

 in Plate XIII. will characteristically 

 represent the phenomena j^rincipally 

 resultant, though the complexity of 

 these phenomena is so great that in 

 Fig. 3. detail they can only be followed out 



by the reader with good specimens of the stones in his hand. 



Fig. 1 is from a very rare agate in my own collection, which 

 unites the characters of the folded group with that of the nested 

 agates which have level beds (the pure folded agates never, as far as 

 I have seen, contain rectilinear tracts), and the folds, or tubes of 

 arrest, in this stone are less regular in structure than in typical 

 examples, and present somewhat the appearance of having been 

 caused by contraction, the rent spaces being afterwards filled by the 

 inner quartz. But I believe this appearance to be wholly deceptive. 

 Whatever the cause of the interruptions may be, they are certainly 

 not mere rents like those of septaria. The greater width of the 

 white band at the top, which suggests the idea of large influx there, 

 is a sectional deception ; this white band is of equal thickness every- 

 where ; and, with all the others, seems entirely concentric, except 

 when interrupted by the tubes, and by the changes in the direction 

 of the films in its own substance which are connected with- them. 

 Fig. 2 is from a piece of perfect folded agate, showing the symme- 

 trical arrangement of its successive beds round the tubes, and their 

 lovely dependent curves as they detach themselves. In some cases, 

 however, the tubes appear isolated in the mass of the stone, or in- 

 terrupt the beds in their own thickness ; but in whatever accidental 

 relation to the secreted chalcedony, they assuredly indicate a peculiar 

 state of its substance at the time of secretion ; and their nature, and 

 the conditions under which they develope themselves, must be under- 

 stood before we can hope to explain the more complex tubular form- 

 ation of dendritic chalcedonies. 



And this investigation is rendered doubly difficult by the per- 

 petual confusion in all agatescent bodies between the concretionary 

 separation, and successive deposit of their beds. If these folded 

 agates were, indeed, formed in successive beds, from without 

 inwards, as it has been supposed, it should be possible some- 

 times to trace the point of influx of material, and the sequence of 

 the added bands from it, which I never yet have been able to do 

 satisfactorily in a single instance in folded agates (and only with 

 suspicion of the appearance of it, even in the brown coated and level 



