154 DREDGINGS OF THE MARINE BIOLOGICAL ASSOCIATION : 



followed. The slide now shows a clear-cut boundary ; on the side 

 which was varnished the foraininifera and shell fragments remain, on 

 the other side they have vanished, leaving absolute vacancies. The 

 crypto-crystalline silica is identical in character on either liand. The 

 included quartz grains, which occupy the whole thickness of the slide, 

 polarise in higher colours than the silica of the ground-mass, which is 

 in too minute form to extend through the whole depth. 



One interesting point is that the chalk thus converted into flint 

 had not the same original structure as the yellow chalks described 

 previously. (Plate X, figs. 1 and 2.) 



Tlie following further specimens were microscopically examined. 



M. 15b. S. 27° W. Edd., 20-3 miles. 



Decomposed flint. Lime not so common except in parts. There 

 are instances of foraminiferal shell entirely replaced by silica. These 

 are best seen by polarised light. In places the grain of the silica in 

 the ground-mass becomes comparatively coarse. 



M. 9h. S. 31° W. Edd., 21-7 miles. 



Black cherty flint, with small light markings. The exterior reduced 

 to a white loose-textured substance with minute brown spots. The 

 varying resistance of the flint to decomposition is to be seen where the 

 extreme outer surface has been chipped off, and small patches of almost 

 unaltered rock are visible. 



Shows the crypto-crystalline structure of the ground-mass very well. 

 Much like other sections of the same material, except that the calcite 

 fragments are larger, and there are iron-stained areas. 



M. 18b. S. 29° W. Edd., 234 miles. 



Decomposed flint. Hydrozoa and shell fragments, mainly in calcite, 

 but some wholly replaced by silica of coarser texture than ground- 

 mass. At one place slight dendritic growth of iron oxide. 



Flint is present in greater or less quantity in every sample which 

 yields pebbles or stones. Without actual count an approximate estimate 

 of the proportion of flint to the whole sample has ])een made in most 

 cases with the following results : — 



A. 105. S.S.W. Bolt Head, 1 mile. 

 Flint in gravel 13 %. 



A. 106. S. Bolt Head, 2 miles. 



Flint in gravel 40 %. 



M. 31. Flint two-thirds of whole. 



M. 32. Thirty-seven pebbles, of wliich twenty are flint. 



M. 36. Nine-tenths flint. 



M. 10. Nine- tenths flint. 



