PETROLOGY OF SOME LIMESTONES FROM THE ANTARCTIC 193 



rock, both matrix and grains, was calcareous, and that subsequently dolomitisation 

 and afterwards silicification occurred. In accordance with this, it is found that in 

 places the matrix remains as dolomite, some of the indifferentiated pellets persist as 

 dolomite, some of the oolitic grains are also entirely preserved in dolomite, and where 

 the grains have been silicified the concentric and radial structures are usually defined 

 by minute rhombohedra of dolomite. On the view that the radial structures were 

 the skeletons of radiolaria, one would have to assume that the siliceous network was 

 first replaced by dolomite, and that later extensive silicification of the rock replaced 

 most of the matrix and filled the spaces between the radial and concentric structures. 

 There is no warrant for this view, and all the evidence points to the original composition 

 being calcareous. 



Staining and acid attack. — Two of the sections were stained with Lemberg's 

 solution with negative results, demonstrating that the minute rhombohedra are 

 dolomite and not calcite. In the case of one section, where the rock had been silicified 

 to a varying extent in different parts, it was found that the structure of the oolitic 

 grains was only faintly visible in some cases, as all the dolomite rhombs had been 

 replaced by chalcedonic cryptocrystalline silica. One half of this section was then 

 attacked by hydrofluoric acid, with the result that the radial and concentric structures 

 of the silicified grains were rendered more clearly visible owing to a differential attack 

 of the acid. The chalcedonic silica of the matrix was also unequally attacked, 

 bringing out a banded agate structure, which was previously not noticed (see Plate II, 

 Fig. 1). Possibly the calcareous matrix surrounding the grains had been dissolved 

 out, and later the silica was deposited in concentric layers round the grains. 



Structure and metasomatism in the oolitic grains. — From the above it will be 

 clear that in the oolitic rocks which have remained calcareous only the concentric 

 structure was seen, while both concentric and radial structures occur in each section 

 of the silicified rocks. The following statement defines the types of structure 

 and the probable order of metasomatic change in the oohtic limestones. The types 

 are separated for clearness of statement, but many of them occur together in the 

 same section, showing that in the same rock metasomatic changes affected separate 

 grains in different degree. 



Purely Calcareous Types 



(1) Oolitic limestones composed entirely of calcite, the oolitic grains showing 

 concentric structure only. The matrix consists of coarse interlocking crystals of 

 calcite. Example: Cloudmaker, Beardmore Glacier, No. 20. 

 (2) Dolomitised oolite. 



(a) Partially dolomitised. Generally the nucleus is alone dolomitised. Con- 



centric structure shown. (No. 20.) 



(b) Completely dolomitised. All structure destroyed. (No. 20.) 



Silicified Types 



In these rocks both matrix and grains have been silicified in varying degree. 



The matrix. — The matrix occasionally retains some dolomite, but for the most 

 part it is siliceous, and ranges from cryptocrystalline chalcedonic silica to a definite 

 quartz mosaic. Attack with hydrofluoric acid, as noted above, develops a banded 

 agate structure in the matrix. It is noted that when both matrix and oolitic grains 

 are silicified the silica of the grains is usually cryptocrystalline and finer in texture 

 than that of the surrounding matrix. 



