120 Q. W. Card—Flexihility of Rocks. 



that the flexibility diminishes with the thickness of the lamina ; 

 pieces of 5 mm. or more in thickness exhibit the peculiarity in only 

 a slij^ht degree, while the lamina3 of 1 mm. or less are very flexible 

 indeed. Here, however, I may mention that the behaviour of 

 specimens obtained from the same slab varied very much. A speci- 

 men of average flexibility and of 1 mm. or less in thickness feels 

 almost leathery, bending over in all directions when held by one 

 side or supported in the middle. A strip — trimmed down with a 

 knife — 5"3 cm. long, 2-6 cm. wide, and 1-5 mm. thick, supported 

 at one end so that 5-3 cm. projected, gave the following results. 

 By its own weight (4:'4: grains) the strip bent until the free end 

 was 1-6 cm. lower than the point of support. Now suspending it 

 verticall\' it straightened itself in a few seconds and, on being turned 

 over and once more supported at the same end, bent as before, 

 but to not quite the same amount. By pressing the free end with 

 the finger it was brought 3"1 cm. below the horizontal, when the 

 strip broke off close to the point of support. Experimenting with 

 another piece ; after a little pressure had been applied to the free 

 end, it straightened when suspended, but, when once more supported at 

 one end in the same position, it assumed by its own weight a position 

 a23proximating to that which it had been before bent by pressure. 



With regard to the cause of flexibility I can only offer the follow- 

 ing suggestions. In the first place, room for internal movement is 

 provided for by the abundance of empty spaces, and in the second, 

 the structure revealed by high magnifjang powers suggests the possi- 

 bility that many of the grains are interlocked in such a waj' as to 

 peinnit of a certain amount of movement upon one another.^ Such 

 an explanation would not be incompatible with the coherency of the 

 rock ; as a matter of fact the coherency is ver3f slight, the material 

 crumbling to pieces with great readiness : moreover, many of the 

 grains are to be bound together in such a way that their margins 

 cannot be made out. Owing to the small size of the grains, it is 

 impossible to demonstrate whether they have such power of move- 

 ment or not. 



The empty spaces have, no doubt, resulted from the removal 

 of carbonate of lime by percolating water. The bedding-planes have 

 afforded the easiest passage, and it is along these planes that the 

 cavities for the most part occur. It would appear as if much of the 

 material so dissolved had been redeposited as calcite in the geodes. 



2. The Cause of Flexibility in " Itacolumite." — Before considering 

 this question, it is very necessary to ai"rive at some sort of an under- 

 standing as to what the rock which has been known as "Itacolumite," 

 " Flexible -Sandstone," "Flexible -Quartz," and a variety of other 

 names,^ really is. Towards the close of the last century a peculiar 

 siliceous rock was noticed in Brazil. Attention was directed to it 

 principally because it often constituted the matrix in which diamonds 

 occurred, but also because thin pieces of it were sometimes found to 

 be flexible. Owing to its occurrence in the mountains of Itacolumi, 



1 See Woodcut, Fig. B., p. 123, infra. ' 



2 Third Report ou the Geognostic Siu-vey of South Carolina, 1818, p. 85, et. seq. 



