194 PETROLOGY OF SOME LIMESTONES FROM THE ANTARCTIC 



(3) The oolitic grains. 



Completely dolomitised oolite, but showing both radial and concentric structures. 



(4) Dolomitised and silicified oolitic grains. 



(a) With both concentric and radial structures. 



(b) With only concentric structure. 



(c) With central structureless silicified part and outer ring of dolomite. 



(5) Silicified oolitic grains in which all dolomite has been replaced by silica. 



(a) With concentric and radial structure. 



(b) With concentric structure alone. 



(c) Spherical in shape, but structureless. 



A rather interesting resemblance is noticeable between these silicified oolitic 

 limestones and certain siliceous and dolomitic oolites occurring among the probably 

 pre-Cambrian dolomites of the Transvaal. Two sections from Krugersdorp, in my 

 possession, show oolitic grains with concentric and radial structures preserved in 

 dolomite more or less completely replaced by chalcedonic silica. The matrix of the 

 rocks is also partly siliceous, partly calcareous. 



P. 282. Stranded Moraine, East Fork, Ferrar Glacier. (Erratic) 



A very fine-grained dense limestone, whether dolomite or calcite cannot be 

 determined, as opportunity for staining was wanting. The interest of this rock 

 depends upon the presence of volcanic fragments and minerals and of fossils (Plate II, 

 Fig. 2). The volcanic material consists of angular, brown, iron- stained fragments 

 and of crystals of biotite. A little quartz is also present as minute fragments. This 

 material may be of detrital origin, but it is suggestive of the intermixture of particles 

 of a submarine tuff in a marine limestone. The fossils noticed are several broken 

 pieces of sponge- spicules. The fragments are each in length about two or three times 

 the diameter of the spicule. In each case the central canal is clearly preserved, and 

 the silica remains in its original colloidal form. A comparison with some sponge- 

 spicules of Tertiary age shows a similar freshness, comparable dimensions, and in 

 each case a refractive index lower than that of Canada balsam. These fragments do 

 not present characters definite enough for identification, and so yield no direct 

 information as to the age of the limestone. As remains of Archo?ocyathince are believed 

 to occur in some of the Antarctic limestones, the question arises whether these spicules 

 may be of Cambrian age. In general form, dimensions, and the relation of diameter 

 of the canal to that of the whole spicule they are similar to some sponge- spicules I 

 found in cherty rocks of the Heathcotian series in Victoria (L. Ordovician or U. 

 Cambrian?). Dr. Hinde kindly examined these for me and confirmed their identifica- 

 tion as sponge- spicules, but could not name them. These Heathcotian spicules are 

 recrystallised to chalcedonic silica. While, then, the preservation of the colloidal 

 character of the spicules in the Antarctic rock suggests that the rock has no great 

 geological antiquity, there is nothing in their form and structure to negative the idea 

 of a Cambrian age for the rock if that is indicated by other evidence. 



No. 15. Cloudmaker, Beardmore Glacier, December 10, 1908. (Erratic) 



A limestone which has been extensively shattered and recrystallised. In con- 

 sequence the rock now consists mainly of large interlocking, frequently twinned, and 

 cloudy crystals of calcite. Staining with Lemberg's solution shows that along the 

 structural cracks in the rock magnesian solutions have entered, with the production 



