190 PETROLOGY OF SOME LIMESTONES FROM THE ANTARCTIC 



No. 12. Dolomitic Limestone. Cloudmaker. (Erratic) 



Two sections of this rock have been stained with Lemberg's solution. The rock 

 is fine-grained, with minute fragments of limestone in a calcareous matrix. Scattered 

 through the rock are unstained areas of dolomite, mostly irregularly distributed, but 

 in places following lines of fracture. A few calcite veins also traverse the rock. The 

 rock therefore is a minutely fragmental dolomitic limestone. 



No. 3. Limestone Breccia. Lower Glacier Depot, Beardmore Glacier, 

 October 12, 1908. (Erratic) 



About twenty rock sections have been examined, mostly slices from one specimen. 

 The general cbaracters are seen in the figure (Plate I, Fig. 1). The rock is strictly 

 a dolomitic breccia, and is interesting from its fossil content, structure, metasomatic 

 change, and origin. 



Structure. — The rock is mainly composed of angular to rounded fragments of a 

 limestone which was completely dolomitised before being broken up. In some 

 sections, however, pieces of a calcareous and micaceous sandstone are seen (206, 207, 

 212), detrital fragments of chert (206, 207), dark cherty shale (212), or silicified oolite 

 limestone (209). Angular fragments of quartz resembling vein quartz are sometimes 

 abundant in the breccia ; while sometimes elastic fragments of dolomite contain 

 water-worn fragments of quartz (212), or even other calcareous fragments (206). 

 Cracks in the original dolomitised and silicified limestone have been filled with calcite 

 before the rock was broken up and water-worn. Subsequently the fragments have 

 been cemented with calcite, the cemented breccia again cracked, and the cracks once 

 again filled with calcite. 



Fossil content. — Several of the sections show traces of organisms. Archo?ocyathince 

 have been recognised, I believe, in some ; but those which I have examined have been 

 so metasomatised that only the dark rims or " ghosts" of the organisms now persist. 



Metasomatism. — The water- worn pebbles of limestone in the breccia show that 

 their earlier history includes two types of metasomatic change involving the formation 

 of dolomite and of a silicified rock. 



Dolomitisation. — Several of the rock sections have been stained with Lemberg's 

 solution so that the dolomite and calcite could be precisely determined. The staining 

 has shown that the matrix of the breccia and the vein fillings, both those formed 

 before the pebbles and those of subsequent date, consist entirely of calcite. The 

 calcareous pebbles, with one doubtful exception, consist entirely of dolomite. The 

 dolomite is frequently cloudy, with central dark areas, and consists of interlocking 

 allotriomorphic crystals. Lining what were cavities in the rock are well- shaped 

 rhombohedra, some of which are zoned with altered chalybite, and occasionally with 

 calcite (3). From the fact that all the calcareous pebbles consist entirely of dolomite, 

 one may draw the conclusion that they have been derived from a limestone which 

 had not been merely locally dolomitised along cracks by later infiltrations of magnesian 

 waters, but had suffered complete regional submarine dolomitisation during the 

 formation of the limestone. 



This observation is of special interest, as it indicates that the limestone was formed 

 in shallow water, a conclusion we are entitled to draw owing to the evidence of the 

 mode of occurrence of dolomite among ancient and modern coral limestones, as 

 recorded by Dr. Cullis and by the author, among others.* The shallow-water origin 



* Cullis, Funafuti Report, 1904. Skeats, Bull. Mus. Comp. Zool. Harvard, vol. xiii. 1903, 

 p. 125. Q.J.G.S., vol. lxi, 1905, pp. 133-137. 



