314 



spherical bodies. Tlie latter differ greatly in size. Some are 

 not very sharply defined from the matrix. The majority are 

 less than one-hundredth of an inch in diameter, while some 

 attain the size of one-twenty-fifth of an inch, or even larger. 

 Many are oval in outline, and in such cases there is a tendency 

 for them to occur with their longer axes all pointing in the 

 «ame direction. These rounded bodies appear to be formed 

 of the same material as the granular part of the matrix. 

 There is an absence of the concentric structure commonly 

 seen in oolitic grains, but the marginal area is usually denser 

 and darker than the rest, probably arising from their 

 spheroidal form and also from the fact that many are filled 

 by crystals of calcite. Many of the spherules also show a 

 faint and dull reddish colour, especially towards their centres. 

 It is this colouring matter in the spherules that gives the 

 dull, pinkish, or reddish colour to the rock in bulk. 



These non-concentric spheroids do not appear to have 

 "been formed by precipitation around a nucleus (unless the 

 crystals of calcite which occupy the central portions of many 

 of them be regarded as such), but they have the appearance 

 of minute pellets of the groundmass, the constituents of which 

 have become agglutinated by a cementing agent. This 

 cementing agent is clearly the ferric oxide that gives colour 

 to the rock, and probably originated by the hydration and 

 oxidation of a diffused iron compound that formed part of 

 the original material of the rock. 



In addition to the minute spheroidal bodies the lime- 

 stone, in certain layers, is studded with yellowish and earthy- 

 looking patches, up to half an inch in length, which on 

 analysis proved to be portions of limestone coloured with 

 ochreous material. Obscure traces of radiolaria have been 

 -detected in some parts of the limestone, (24) and having 

 recently forwarded some etched surfaces of the limestone to 

 Mr. F. Chapman (Palaeontologist, National Museum, 

 Melbourne), he courteously informs me, 'The majority of the 

 siliceous spheres are nearly twice the usual diameter of the 

 Barbados and living radiolaria, although some recent forms 

 are even much larger. They average about '2 mm. in 

 diameter. In more than one I could detect the radial bars 

 and another the central shell. The mesh work, although 

 irregular, could hardly belong to anything outside the 

 radiolaria, and their condition suggests that there has been 

 a tendency for the structure to break down during the 

 silicification of the matrix. The smallest rock specimen carries 



(24) D,avid and Howchin, ''Note on the Occurrence of Casts 

 of Radiolaria in Pre-Caimbrian ( ? ) Rocks of South Australia," 

 Proc. Linn. Soc. N.S. Wales, 1896, part 4, p. 571, pis. xxxix., xl. 



