160 Buskin — On Banded and Brecciated Concretions » 



the average the third of an inch in diameter, — but the lines of division 

 are so subtle that the mass appears compact) . In this character the 

 inner deposit seems only a finer condition of the external one : but it 

 differs specifically in being affected by sharp displacements apparently 

 owing to contraction. To these faults, though minute, I would 

 direct the reader's special attention. They are by no means small 

 in proportion to the extent of material affected by them ; and they 

 differ wholly from ordinary displacements, in this, that there is no 

 trace whatever of movement at the limiting convex curves, but only 

 at the edge of the chalcedony — so that the fault at a seems owing to 

 contraction within the space a b, and at c, to contraction within little 

 more than the space c d ; and farther, the fissures ah, c d are not 

 rugged or broken, as if caused by the displacement, but sinuously 

 current, passing on through the chalcedony from c to e,f, and g ; and 

 in fact, I am very certain that these veins are not caused by the con- 

 traction in question ; but that the contraction takes place unequally 

 on each side of the primarily formed vein. This kind of faiilt, of 

 which we shall find frequent instances, the unequal contraction, 

 namely, of beds on opposite sides of a vein or dyke, I shall call fault 

 " by partition," and the violent fracture of beds at a point where no 

 vein or dyke previously existed, I shall call fault '' by divulsion.'* 

 Deposits which fill compartments in fossil shells may often 

 be seen, in a correspondent series of beds, to vary their pro- 

 portionate thickness at each partition : the rectilinear bands of 

 Labradorite may be found varying in thickness and position while 

 they correspond in direction, in contiguous crystals ; and I do not 

 doubt but that even on a great scale, displacements of beds which at 

 first sight might be supposed to have given rise to the fissures which 

 divide them, will be found on examination to be the result of an 

 unequal contractile action in the masses released by the fissure, pro- 

 tracted for long periods after it had given them their independence. 

 Lastly. The separation of the chalcedony from the jasper does not 

 take place only in the inner formation. It is an operation evidently 

 subsequent to the deposition of both layers, and even in the outer 

 one, makes the entire dotted space, as far as the curved limit x y , 

 chalcedonic, and flushes it with a diffusion of the medial oxide from 

 its edges ; this medial oxide here drawing itself into bands, which 

 being parallel with those of the grey chalcedony, are manifestly pro- 

 duced by a segregation which has taken place simultaneously in the 

 two layers. This being clearly ascertained, the intensely sharp line, 

 which separates the chalcedony from the while jasper, considered as 

 a result of segregation, becomes highly remarkable, and a standard 

 of possibility in sharpness of limit so produced. 



The spots surrounded by dark lines in the lower part of the figure 

 are portions of the inner formation cut off by the surface section. 

 It is often difficult on a single plane to distinguish such spaces, the 

 truncated summits of an inferior, or, as here, remnants of a superior, 

 bed, from isolated concretions : and it is always necessary in examin- 

 ing agates to guard against mistaking variation of widths of belt 

 caused by obliquity of section from true variations in vertical depth. 



