190 PETROLOGY OF SOME LIMESTONES FROM THE ANTARCTIC 



The bulk of the rock is a colourless groundmass which in polarised light consists 

 of a cryptocrystalline to microcrystalline arrangement of crystals polarising in neutral 

 tints. Some are rectangular in outline, and may be felspar ; the majority are irregular 

 in shape, and consist probably of chalcedonic silica. The rock presents considerable 

 resemblance to a fine-grained spilosite, but may be a dense ferruginous chert. 



Limestones in situ from Farthest South 



Two specimens brought back from Farthest South owe their importance to their 

 geographical position, to the fact that they were taken from rocks in situ, and to 

 the fact that the discovery in them of Archceocyathince fixes their age as Lower 

 Cambrian. 



No. 106. Farthest South.* December 20, 1908. (In situ) 



Six sections of this rock have been examined, five mounted in the ordinary way, 

 the sixth after staining with Lemberg's solution. The hand-specimen is a grey, 

 dense limestone, with a few lighter calcite veins and specks of pyrite scattered 

 through it. 



Under the microscope staining shows that the rock consists chiefly of calcite, with 

 a subordinate development of dolomite. For the most part the dolomite is developed 

 in sporadic crystals throughout the rock. In places, however, it occurs along cracks, 

 and at one point a large number of well- developed rhombohedra are grouped 

 together with a calcite background. The rhombohedra of dolomite have cloudy 

 centres and clear margins. This cloudy material is for the most part dolomite, since 

 it is unstained ; but in two cases the kernel of the crystal consists of calcite and the 

 margin of clear dolomite in optical continuity with the calcite. The rock is slightly 

 mineralised, as cubes of pyrite are sparsely scattered through it. There are faint 

 indications of the former presence of organisms, and structurally the rock has been 

 subjected to considerable earth movements. Some of the cracks lined with dark 

 material or with dolomite crystals show a very tortuous passage through the rock, 

 indicating later squeezing. The latest movements induced straighter cracks, now 

 filled with clearer, larger crystals of calcite than in the matrix, which consists of 

 somewhat cloudy allotriomorphic calcite crystals. 



No. 107. Farthest South, December 20, 1908. (In situ) 



The hand-specimen is a dense, fine-grained rock with irregular streaks of a green 

 material distributed through it. 



Two sections have been examined, of which one was stained with Lemberg's 

 solution (Plate II, Figs. 3 and 4). No dolomite can be recognised, the whole of the back- 

 ground of the rock consisting of minute greyish granules of calcite, except for a few 

 micaceous particles. Obscure traces of fossils, including Archceocyathince, can be 

 seen. A point of considerable interest is the nature and origin of the material, which 

 has a greenish appearance in the hand-specimen. Under the microscope it is very 

 fine-grained, and is barely resolved under a {,-inch objective. 



Chemical composition. — With the idea that a chemical analysis would help in the 

 elucidation of this question, I asked Mr. F. L. Stillwell, B.Sc, Kernot Eesearch 

 Scholar in Geology in the University of Melbourne, to undertake this for me. I am 

 much indebted to Mr. Stillwell for the care and time he has given to this work. He 



* This refers to the "farthest south" outcrop of rock in this vicinity; not the farthest south 

 point reached by the party. 



