40 DE. G. J. HTNDE ON THE EADIOIARTA IN THE [Feb. 1 899, 



but very slightly, if at all, altered ; the casts of radiolaria are 

 similar to those in the chert described above, and the groundmass 

 has less of the dark carbonaceous (?) material and a somewhat 

 larger number of very minute polarizing particles. 



In general characters the radiolarian chert from these Australian 

 localities very closely resembles that from Mesozoic and Palaeozoic 

 formations in other parts of the world. The radiolaria in it are too 

 poorly preserved for either generic or specific determination ; one 

 can only say that the rock is filled with their casts, and that the 

 silica of the chert has most probably been derived from their tests. 



[b) The Siliceous Limestones with Radiolaria. 



The examples of this rock which I have examined are all from 

 Tamworth, and they bear the following numbers : — 245 3, 247 3-4, 

 248 4, 256 8, 4-4 a. The rocks are dark to a dull black, fine- 

 grained, compact, homogeneous. They weather on the outer 

 surface to a porous brown crust, having much the appearance of 

 rotteustone. In only one instance (248 4) is the rock traversed by 

 minute veins, and these are partly filled with calcite and partly 

 with microcrystalline silica. The rock readily effervesces in acid, 

 and leaves a brown residue in which there may be seen fairly perfect 

 radiolaria in whitish silica, with spines and central tests. Earely, 

 however, are the organisms completely freed from the porous brown 

 matrix, and they are altogether too fragile to be manipulated for 

 examination, so that for this recourse must be had to microscopic 

 sectious. 



In thin microscopic sections of this dark limestone, made in the 

 usual manner, the radiolaria are very imperfectly shown ; as a 

 rule, only the outlines of circular and oval forms, without spines, 

 are visible, and the tests and the interspaces of the groundmass are 

 filled with somewhat dirty-looking calcite. On removing, however, 

 the calcite by etching the thin section in position on the glass slide 

 with dilute acetic acid for a short time, and after gently washing it 

 and covering it cautiously with balsam and thin glass, the radiolaria 

 are shown with remarkable distinctness. In many of the tests, which 

 range between 0-06 and 0*2 mm. in diameter, the radial spines and 

 the inner or medullary tests are perfectly preserved ; in these latter, 

 as a rule, the delicate latticed structures are shown more clearly 

 than in the outer or cortical tests. The tests and spines are, not 

 infrequently, as transparent as in recent or Tertiary forms, and, 

 like these, they retain the original amorphous condition of the silica. 

 But more generally the original tests are now replaced by a dark 

 substance, as if some compound of iron had united with the silica. 

 Also many of the radiolaria are now represented only by hollow 

 casts, their tests having been dissolved. 



In the best preserved of the etched sections, the rock — minus the 

 calcite — appears to be wholly composed of the tests of radiolaria, 

 crowded together and embedded in a confused mass of broken spines 

 and fragments of lattice-work, the debris of forms which have gone 



