EXPLANATION OF THE PLATES 



PLATE I 



Figure 1. — No. 3. x 5. Dolomitic breccia with Archceocyathince, Beardmore Glacier. 



Eounded and angular fragments of dolomitised limestone with angular and 

 crushed quartz fragments are set in a matrix of calcite. Later veins of calcite 

 traverse the larger fragments and the matrix. 



Figure 2. — No. 20. x 12. Dolomitic oolitic limestone. 



Several grains show good concentric structure, one grain in the centre of the field 

 is largely replaced by dolomite and three defined rhombohedra of dolomite have 

 invaded the concentric layers of a grain near the upper part of the field. The 

 matrix is chiefly clear calcite. 



Figure 3. — No. 245. x 12. Silicified oolite from erratic at Cape Eoyds. 



The grains show both concentric and radial structures. One composite grain is 

 seen near the lower part of the field. The substance of the grains is chiefly dolo- 

 mite with partial replacement by metasomatic silica. The groundmass is partly 

 dolomite but sporadic clear areas of chalcedonic silica indicate partial metasomatic 

 replacement. 



Figure 4. — No. 240. x 28. Silicified oolite from erratic at Cape Royds. 



A good example of a dolomitised grain showing radial structure, another con- 

 centric and a third composite are seen. An irregular pellet, dolomitised and 

 silicified, has its boundary defined by a dark layer. The matrix is now mainly 

 chalcedonic silica. 



PLATE II 



Figure 1. — No. X. x 28. Silicified oolite from erratic at Cape Royds. 



Several stages in the silicification of dolomitised oolitic grains are seen. The 

 original matrix has been mainly replaced by chalcedonic silica. Attack by hydro- 

 fluoric acid has affected the silica unequally and has developed an agate structure 

 between the individual grains. 



Figure 2. — P. 282. x 277. Limestone from Stranded Moraine, East Fork, Ferrar 

 Glacier. 



A fine-grained limestone with two or three fragments of sponge spicules. One 

 spicular fragment showing the central canal is seen above the centre of the field. 



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