BY T. W. EDOEWORTII DAVID. 559 



The radiolariau beds are comj)Osed of dark to black chert with 

 a hackly fracture. Other portions are dull grey to white, or the 

 rock is made up of alternate light and dark bands, so as to be 

 striped. 



In places the rock is platy, siliceous, or mottled white and 

 black. The soft gi'ey to white beds are very rich in radiolaria. 

 They disintegrate in some cases in water into a fine cream-coloured 

 mud. 



The soft beds are of much less frequent occurrence than the 

 hard cherts. 



The individual radiolarian beds are minutely laminated. 



Microsco/jic character. — Carbonate of lime is conspicuous by its 

 absence. The radiolarian rock generally shows a siliceous ground 

 mass, in some cases clear and transparent, in others dark and 

 turbid from the presence of fine particles of carljonaceous or 

 ferrous minei-als, and minute crystal needles of rutile and zircon. 

 The siliceous groundmass shows between crossed Nicols the faint 

 speckled appearance of cryptocrystalline silica, like Hint from 

 chalk. When radiolaria are abundant chalcedonic tints prevail. 

 The radiolaria in the rock have been filled with clear nearly 

 transparent silica free from the rutile crystals and from the dark 

 substances disseminated in the groundmass, and either micro- 

 crystalline or cryptocrystalline. Within the radiolarian casts 

 the silica is often fibrous radial, and so show? a black cross in 

 polarized- light. 



The more distinctly crystalline character of the radiolarian 

 casts facilitates their recognition in the rocks with a clear ground- 

 mass where in ordinary light they are scarcely visible, but between 

 crossed Nicols they appear as so many circles of speckled or bright 

 light on a nearly dark ground. 



Minute casts of rhombohedral crystals are frequently present, 

 probably of calcite or dolomite, sometimes inside the radiolarian 

 casts. A similar occurrence has already been referred to in the 

 Hartz Mountains. Microscopic cubes of iron pyrites are present 

 in some of the rocks. 



