192 PETROLOGY OF SOME LIMESTONES FROM THE ANTARCTIC 



seen. In a few cases the whole of the grain, periphery as well as nucleus, has been 

 completely converted into dolomite. 



These areas show up well by their freedom from stain after treatment of the rock 

 with Lemberg's solution. In places, however, these large crystals of dolomite include 

 minute areas of unaltered calcite, as is shown by the slight difference in refractive 

 index and by the staining of such areas. 



Silicified Oolitic Limestone. Cape Royds. (Erratic) 



Nine sections labelled as above have been examined ; seven of these have been 

 sliced from the same specimen, and two, numbered P. 86, have different characters. 



P. 86. — The two sections examined consist entirely of chalcedonic silica and 

 dolomite rhombs. The dolomite rhombs are arranged as minute irregular rings, the 

 long ones ranging from about -^ to I mm. The central parts of the rings and the 

 matrix between them consist of chalcedonic silica. There can be no doubt that the 

 rock was originally calcareous, and afterwards at any rate partially dolomitised. 

 Possibly parts which remained as calcite were then dissolved away, and secondary 

 chalcedonic silica filled the spaces so left. A few calcareous centres to the rings of 

 dolomite are to be seen, but the remainder of the ring-centres in ordinary light show 

 a colourless and structureless infilling of silica. The origin of these ring-shaped areas 

 is doubtful. They may represent remains of oolite grains distorted in shape after 

 the removal by solution of the central areas, but they are of much smaller dimensions 

 than the undoubted oolites, and no positive opinion can be expressed. 



The remaining seven sections (Nos. 222, 233, 240, 244, 245, and two un- 

 numbered ones) all show fairly uniform characters, which are seen in Plate I, Figs. 3 

 and 4. 



General characters. — They are all dolomitised and silicified oolitic limestones. The 

 average diameter of the grains is about \ mm., while that of the grains from Cloud- 

 maker is, as previously noted, about 4 mm. 



A few irregular, structureless pellets occur, some of which consist of dolomite, 

 others are partially or wholly silicified. A few composite oolitic grains occur, but 

 the large majority are simple, and nearly spherical in shape. In these silicified rocks 

 a few of the oolite grains show the concentric structure only, the great majority 

 having a central nucleus, an oiiter margin showing concentric structure, and a middle 

 layer with a well- pronounced radial structure. 



Comparison with radiolaria. — So closely do these latter oolitic grains resemble 

 radiolaria on first inspection that I have paid particular attention to them and 

 instituted careful comparisons with recent and fossil radiolaria. I have been led to 

 abandon the idea that they might be radiolaria for the following reasons : 



(1) Size. — The average diameter is about 5 mm., while the diameters of most of 

 the recent and fossil radiolaria I have measured ranged between -08 and "3 mm. 



(2) Structure. — (a) There is no trace of the reticulated or mesh structure as seen 

 in the skeletons of radiolaria. 



(b) Instead of a uniform series of concentric layers at fairly wide intervals, as is 

 usual in many radiolaria, these grains show commonly several closely adherent layers 

 round the outer margin of the grains — a feature characterising undoubted oolitic 

 grains. 



(c) The radial appearance seen is probably clue to crystallisation and not to organic 

 structure, for in the case of the composite grains not only is it seen in the individual 

 grains but it occurs also in the outer part of the composite grain. 



(3) Composition. — There can be little doubt that the original composition of the 



