SPECIMENS OF CHIASTOLITE FROM BIMBO WRIE — ANDERSON. 301 



from weathering. Only two of the clay-slate wedges are affected 

 in this way. The central inclusion figure is very small in this 

 specimen. 



Figs. 4 and 5. — Here the weathering effect is still more pro- 

 nounced, the transverse section resembling a penetration twin. 

 That the form is not the effect of twinning is apparent, since it 

 differs in no respect from an ordinary crystal, save that the 

 included wedges of clay-slate have been removed. Fig. 5 gives a 

 longitudinal view, from which it is seen that the surface is 

 homogeneous, presenting no clay-slate wedges, yet the transverse 

 section shews the remains of the dark triangular areas, correspond- 

 ing no doubt to the inner areas of inclusions in fig. 2. Fig. 5 

 also shews a peculiar feature which I have not seen alluded to in 

 any description of chiastolite. Nine specimensshew this structure 

 out of thirty eight examined, hence it is a well-marked feature of 

 the Birabowrie mineral. In all cases where the removal of the 

 clay-slate has proceeded to a considerable extent, this phenomenon 

 occurs, and the appearance in all cases is strictly analogous. The 

 cross has two longer and two shorter arms, the former alone 

 shewing this peculiarity, which consists of a series of grooves, 

 alternating with ridges, which proceed from the junction with the 

 shorter arms to the boundary of the crystal, and these grooves and 

 ridges are always curved in opposite directions, as shewn in fig. 5, 

 where those on the right side have the concavity downwards, 

 while those on the left have the concavity upwards. In most 

 cases another peculiarity is noticeable, namely, the longer arms 

 are slightly bent in opposite directions, as if a shearing stress had 

 acted on the crystal in a direction perpendicular to the vertical 

 axis. Thus, in fig. 5, the right side is bent away from the 

 observer, the left side towards him. This deformation is well- 

 marked whenever the grooving is most apparent. In one speci- 

 men the arms are nearly straight and the grooving is but slightly 

 developed, hence it may be that the two phenomena are related. 

 In two cases the clay-slate wedge has persisted in alternate 

 reentrant angles, and, assuming the action of the stress referred 

 to, the phenomenon in these two cases may be not inaptly com- 

 pared to " crag and tail " of geologists, the wedge being scooped 

 out on the side exposed to this hypothetical force, while it still 

 remains on the other. 



Fig. 6 furnishes a notable example of parallel growth. The 

 foreign matter occupies roughly parallel positions in the two 

 individuals, while one patch of inclusions is common to both. 

 The included areas are of uniform dark color, and shew no differ- 

 entiation such as is apparent in fig. 2, and the crust is homogeneous. 



Rohrbach^ explains the peculiar structure of chiastolite crystals 

 by supposing the growth to proceed unequally, the crystal 



2 Eohrbach— Zeitschr. Deuts. Qeol. Ges., 1887, 39, 635. 



