PETROLOGY OF SOME LIMESTONES FROM THE ANTARCTIC 195 



of crystals of dolomite more or less confined to the fractured areas. Still later cracks 

 have been healed by the deposition of clear twinned crystals of calcite. 



P. 301. Marble. Stranded Moraine. (Erratic) 



The rock is a recrystallised limestone. It consists now mainly of large, inter- 

 locking, twinned crystals of calcite, one or two pieces of quartz, some opaque 

 fragments of graphite and of pyrite altering to limonite, and a number of small 

 colourless crystals of talc. The talc is recognised by having a fairly low refractive 

 index, high polarisation colours in prismatic sections, by giving a pseudo- uniaxial 

 and negative interference figure, and by its micaceous habit. The talc may have 

 been developed in the rock during recrystallisation, and may be the product of 

 dedolomitisation of a slightly siliceous and magnesian limestone. The quartz and 

 opaque material, however, appear to be detrital, and the talc may have a similar 

 mode of origin. 



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P. 305. Dry Valley, Antarctic. (Erratic) 



A carbonate forms the bulk of this rock. The section was mounted unstained, so 

 it remains uncertain whether the mineral is calcite or dolomite; probably it is the 

 former. Several detrital grains of olivine, more or less altered to serpentine, are 

 scattered through the rock. Minute crystals of pyrite are also present. Reddish- 

 yellow ferric hydroxide occurs in irregular areas or surrounding opaque masses of some 

 iron mineral. Irregular patches of graphite are noticeable in places. In addition 

 there are scattered through the rock a number of irregular- shaped sections of a 

 colourless mineral with a refractive index a little higher than that of Canada balsam. 

 The majority of the sections show no cleavage and low polarisation colours; while 

 the sections showing cleavage exhibit bright colours. In convergent light pseudo- 

 uniaxial or slightly biaxial figures are seen. The sign is negative. The mineral is 

 therefore almost certainly talc. The olivine in the rock is certainly of detrital origin, 

 and its presence indicates that the limestone formed near a shore-line consisting in 

 part of basic igneous rocks. The talc may be detrital, but from its irregular 

 boundaries with the limestone may have developed in the rock by the alteration of 

 a magnesian mineral. The rock is a limestone of shallow- water origin. 



Kl. Dunlop Island ; between Cape Bernacchi and 

 Granite Harbour. (Erratic) 



This is a very fine-grained rock, consisting of minute allotriomorphic granules of 

 a single material, which is probably dolomite. 



P. 268. Dunlop Island. (Erratic) 

 This section is exactly similar to Kl described above. 



No. 4. Detrital Rock: Cloudmaker, December 10, 1908. (Erratic) 



This rock is not a limestone, but an altered detrital rock. The hand-specimen 

 is very hard and compact. Under the microscope irregular minute yellow- brown 

 specks of hydrated oxide of iron are distributed through the rock, and a few irregular 

 grains with fairly high refractive index and bright polarisation colours consist probably 

 of a pyroxene. 



