RECENT ADVANCES IN SCIENCE 39 



what similar conclusions are reached by A. Scott (Trans. Cer. 

 Soc. 16, pp. 137-52, 191 7). A. F. Rogers (Amer. Journ. Sci. 

 (4) 45, pp. 222-6, 191 8) describes two natural occurrences of 

 cristobalite in rocks from California, the mineral in one case 

 being paramorphous after tridymite. 



In a discussion of the origin of flint, E. R. Lankester (Nature, 

 99, pp. 283-4, 191 7) considers that it is formed at a period 

 long subsequent to the deposition of the chalk, and that the 

 main agent is percolating atmospheric water containing silica 

 in solution. The colour of black flint, which is supposed to 

 consist of minute quartz crystals cemented by amorphous 

 silica, is ascribed to carbon. On the other hand, W. A. Tarr 

 (Amer. Journ. Set. 44, p. 409, 191 7) inclines to the view that, 

 so far as certain Lower Carboniferous rocks in Missouri are 

 concerned, the flint has formed at an early stage, and is not 

 a replacement of the calcium carbonate. B. Moore (Nature, 

 99, p. 324, 191 7) believes that the flint is precipitated from 

 colloidal carbonated solutions during the interaction of the 

 calcite and the carbon dioxide to form " acid " calcium car- 

 bonate. " Synthetic flint," which, however, is much less 

 hard than the natural material, has been obtained in this way. 

 S. C. Bradford (ibid. p. 324, 191 7) considers the formation due 

 to crystallisation from gels, while G. A. J. Cole (Geol. Mag. (6) 

 4, pp. 64-8,^917) adopts a theory analogous to that of Liese- 

 gang, and holds that many flints are formed by the rhythmic 

 precipitation of silica from solution. 



J. V. Samoilov (Min. Mag. 18, pp. 87-98, 191 7) ascribes 

 the presence of certain minerals, barytes, celestite, etc., in 

 some sedimentary rocks to the action of marine organisms 

 which concentrate the salts from sea water. 



R. A. Daly (Proc. Nat. Acad. Sci. 3, pp. 659-65, 191 7) 

 reviews previous work on the low temperature formation of 

 felspars, and describes a new occurrence in a dolomite in the 

 Rocky Mountains which seems to be analogous to the occurrence 

 of felspar in certain European Jurassic rocks. He concludes 

 that the felspar must have been precipitated on the sea floor 

 at a temperature under ioo°, but gives no theory as to the 

 chemical conditions. 



H. Reis (Amer. Journ. Sci. 44, pp. 316-18, 191 7) describes 

 a peculiar clay from Texas, which, under the microscope, is 

 seen to be almost wholly composed of minute crystals of 



