632 DR. G. J. HINDE AND MR. HOWARD POX ON [Nov. 1 89 5, 



phthanites of Britanny which have been described by M. Cayeux 1 

 as radiolaria and foraminifera. On this account we have carefully 

 examined them under the microscope, with the kind assistance of 

 Mr. Allan Dick. The bodies in question are now either hollow or 

 filled with colloid silica ; their walls are of an amber tint in trans- 

 mitted light, with bright specks here and there, in some cases 

 resembling perforations. They are apparently formed of a flocculent 

 material without definite outlines, the nature of which is doubtful, 

 but Mr. Dick suggested that it might be a film of limonite remaining 

 after the removal of pyrites. Some of these bodies occur within the 

 radiolaria, in the same manner as the larger cubes of pyrites and 

 the rhombic crystals. There is no evidence to show that they 

 were of organic origin. 



The finely laminated character which is so marked a feature of the 

 radiolarian rock is seen under the microscope to be due to regular 

 lines of dark amorphous material and finely granular particles, pro- 

 bably in part carbon and in part a compound of iron. In the darker 

 varieties of the rock this material is so abundant that, save in 

 extremely thin sections, the rock is opaque ; but in the lighter and 

 harder varieties it is arranged in alternating bands or laminae from 

 •12 to 5 mm. in thickness. The radiolaria are more conspicuous in 

 the darker bands or laminse than in the lighter, but they are not 

 limited to any particular variety of the rock, and they occur both in 

 the very hard and cherty and in the soft shaly beds. 



Thin sections of the comparatively soft shaly radiolarian beds are 

 not materially different under the microscope from the hard cherty 

 varieties. The siliceous matrix is similarly cryptocrystalline ; it 

 contains minute cubes and grains of pyrites and needles of rutile, 

 and there are also in it numerous small flakelets of miea. In some 

 beds there are dark parallel lines crossing obliquely the lines of 

 bedding, and between them the radiolaria are often closely packed 

 in regular layers. The radiolaria in these softer beds are, as a 

 rule, in the same mineral condition of solid casts of microcrystalline 

 silica as in the harder rock, and owing to the soft nature of the 

 matrix these solid casts weather out and project, as already men- 

 tioned, above the general surface of the rock. In some beds of the 

 soft shales the radiolaria are now represented by empty casts, 

 or these are filled with loose powdery material, and the rock has a 

 light and porous character resembling that of rottenstone. 



Some of the shales are sufficiently soft and unconsolidated to 

 disintegrate in water to a milky sediment ; the finer portions of 

 this consist of minute siliceous granules, flakelets of mica, and 

 rutile-needles ; in the coarser portions are detached radiolaria, but 

 these are usually partially incrusted with the flaky matrix. Under 

 favourable conditions of light, the lattice-structure can be indistinctly 

 seen in some of these detached forms. 



Beyond radiolaria and fragmentary sponge-spicules there is a 



1 Bull. Soc. geol. France, ser. 3, vol. xxii. (1894) p. 197 ; Comptes-rendus 

 Acad. Sci. Paris, vol. cxviii. (1894) p. 1433. 



