PETROLOGY OF SOME LIMESTONES FROM THE ANTARCTIC 191 



of the rock is also confirmed by the presence of detrital quartz fragments in some of 

 the dolomite pebbles (212), and by the fact that some of the pebbles show well-defined 

 oolitic structure (209), which, whether of organic or inorganic origin, is confined to 

 limestones of shallow- water origin. 



Silicification. — Some of the dolomite pebbles and fragments have also been 

 silicified. The introduction of silica followed and did not precede dolomitisation. 

 This is clear from a study of the mode of occurrence of the silicified areas. These are 

 frequently irregular patches, appearing sometimes to fill up cavities in the rock which 

 had been lined by perfectly shaped rhombs of dolomite. In one section of the dolomite 

 breccia a fragment of a silicified and dolomitised oolitic limestone (209) occurs. 

 Replacement of the dolomitised oolitic grains and of the matrix by chalcedonic silica 

 is almost complete. A few rhombs of dolomite remain; the outlines of the oolitic 

 grains are ill defined, and later cracks through the pebble, formed since it reached 

 its present position, have been filled with calcite. 



History. — A study of this dolomite breccia and its components suggests the 

 existence in the area whence the fragments were derived of apre-Cambrian series of 

 igneous rocks, acid, and probably also basic and also of micaceous and calcareous sand- 

 stones. The Cambrian sea received fragments of these rocks along the shore- line 

 while the shallow- water Archwocyathince limestones were being laid down. Dolomitisa- 

 tion and silicification of the limestones occurred, and at a later stage they were 

 elevated, disintegrated, seamed with veins of calcite and subsequently denuded. They 

 then provided the materials for a dolomitic breccia which, after consolidation by a 

 calcite matrix, was traversed by differential movements, forming cracks which were 

 again filled with calcite. 



THE OOLITIC LIMESTONES 



The presence of silicified oolite pebbles has already been noted in the description 

 of the dolomite breccia. Further evidence is obtained from erratics at two localities 

 at Cloudmaker. on the Beardmore Glacier, and also from among the erratics of Caps 

 Royds. 



No. 20. Oolitic Limestone. Cloudmaker, Beardmore Glacier. (Erratic) 



Three sections of this rock were examined, of which two were stained with 

 Lemberg's solution. Its appearance is shown in Plate I, Fig. 2. The rock shows 

 the very well-preserved oolitic or pisolitic structure, the grains being of large size, 

 ranging up to 4 mm., or about |tk inch in diameter. The majority of them are 

 spherical in shape, but some are elongated, and others have been fractured. One or 

 two examples of double oolitic grains were seen. The central part of the grain appears 

 usually to have been recrystallised, and consists now of large interlocking crystals, 

 mostly of calcite. No radial structure is noticeable, but the concentric layers are 

 well preserved. The layers appear to consist of minute tubes j which are occasionally 

 not arranged concentrically, and this would appear to support- the view that these 

 large oolitic or pisolitic grains are of organic origin. The matrix of the rock, like the 

 centre of the grains, has been recrystallised to a calcite mosaic. The rock also shows 

 partial dolomitisation. Much of the dolomite appears to be sporadic in occurrence, 

 but some crystals are definitely placed along cracks in the rock. 



Except along cracks, the general matrix of the rock has not been dolomitised. 

 The oolitic grains, particularly their central parts, are noticeably dolomitised, the 

 salient angles of the rhombs in places advancing into the marginal part, which shows 

 the concentric structure. All stages in the replacement of the oolitic grains can be 



