J . B. Scrivenor — Radiolarian Rocks, East Indies. 245 



It suggests a septarian nodule, but is dull black in colour, and in 

 section shows a brown isotropic base crowded with Radiolaria, sponge 

 spicules, and pyrite. The Radiolaria are mostly ealcite casts, but 

 some are partially replaced by pyrite. 



Comparison of the above-mentioned Rocks. 



Apart from the Radiolarian nodule in shale on the Sarawak E,iver, 

 the origin of which I am at a loss to explain, it would appear that 

 the rocks in Sambas and the rocks in Sarawak containing Radiolaria 

 were deposited under similar conditions, and probably in the same 

 sea, not far from the coast. None of the Sarawak rocks, however, 

 excepting the nodule, can be cited as examples of rocks composed 

 chiefly of Radiolaria. They are not Radiolarian rocks, but rocks 

 containing some Radiolaria, mingled with abundant other organisms 

 in the case of those associated vpith the limestones. 



This, however, is not the case with the Central Borneo and 

 Peninsular cherts, which are distinctly Radiolarian rocks. Professor 

 Molengraaf claims that those in Central Borneo are oceanic, abyssic 

 sediments, but as regards tliose in the Peninsula I do not see any 

 good reason to believe that the cherts vs^ere laid down in an abyss of 

 the ocean, but think rather that a sudden change in conditions apart 

 from depth led to the establishment of a flourishing colony of siliceous- 

 shelled organisms in a sea where hitherto limestone rich in corals and 

 Cephalopods and other calcareous shells had been forming. These 

 cherts are not silicified limestones. 



On comparing these two occurrences of Radiolarian rocks, however, 

 we find that there is a considerable lithological difference. Professor 

 Molengraaf thus describes the Central Borneo rocks : — 



" The first type is the Radiolarian bornstone, the true Radiolarite. 

 This rock is semi-transparent, hard, brittle, and splintery, and of 

 a colour varying between milkwhite, red, and greenish. It consists 

 of about 97 per cent of silica and is composed almost exclusively of 

 closely-packed tests of Radiolaria, joined together by a siliceous 

 cement. The red-coloured varieties may be called Radiolarian jasper ; 

 they contain a small percentage of iron. The tests of the Radiolaria 

 are difficult to distinguish under the microscope in the milkwhite 

 varieties ; they can be much, better distinguished in the red jasper 

 varieties, but best of all they are preserved and discernible in the 

 greenish varieties of the Long Keloh in the Berau district. 



" The second type is an argillaceous chert or siliceous clay-shale, 

 which is always coloured bright red and contains more clay and iron 

 and less silica than the first type. Radiolaria occur in it in variable 

 quantity, but always much less than in the first type. 



*' The two types pass one into the other quite gradually, and all 

 sorts of intermediate types occur. The distinct stratification of the 

 cherts of the Danau formation is caused by thicker strata of pure 

 Radiolarian hornstone alternating with thin layers which contain 

 more clay and less Radiolaria. 



"Both types and all intermediate stages agree in this respect, that 



^ G. A. F. Molengraaf, " On Oceanic Deep-sea Deposits of Central Borneo " : 

 Proc. Eoy. Acad. Amsterdam, vol. xii, pp. 142-3, 1909. 



