THE PETROLOGY OF SOUTH GEORGIA. 827 



the Cape George Series is decidedly calcareous. These calcareous rocks correspond 

 to Thurach's " granular limestone" and " kalk-phyllit." 



The scarcity and gritty character of the limestones point to their inconstant and 

 local deposition in an offshore area in which the dominant sedimentation was aren- 

 aceous in character. The whole series, in fact, except perhaps some of the shales, is 

 indicative of shallow-water origin in a sea bordering a coast on which volcanoes 

 emitting dominantly trachytic lavas were intermittently active. The presence of 

 radiolaria in many of the rocks does not invalidate this conclusion, for radiolaria are 

 now known to occur in many rock-types of undoubted shallow-water origin.* 



II. The Tuffs. 



Megascopically the tuffs are hard compacted rocks of a dark grey to black 

 colour, in which numerous small rock and mineral fragments can be distinctly seen. 

 They closely resemble certain coarse greywackes. In thin section they are seen to 

 consist of broken crystals and rock-fragments embedded in a fine-grained paste 

 (Plate XCIV, fig. 3). Amongst the crystals the most abundant is orthoclase, frequently 

 mottled in polarised light as to suggest the presence of the albite molecule. Various 

 plagioclase felspars also occur. The commonest varieties are oligoclase, extinguish- 

 ing at 5° to 10°, and andesine, with a maximum extinction of about 20°. There 

 are occasionally a few sporadic grains of quartz. A remarkable feature is the almost 

 complete absence of mafic (ferromagnesian) minerals. Only a few small fragments 

 of a pale brown augite and specks of iron-ore have been noted. The augite is 

 occasionally enclosed in orthoclase, and forms with it small glomeroporphyritic 

 aggregates. The only other mineral of importance is scapolite, which has resulted 

 from the alteration of the felspars, a process dealt with later. 



The great mass of the rock is built of small subangular to rounded fragments of 

 igneous rocks. The majority of these are from 1 to 2 mm. in diameter, but range 

 up to 6 mm. Most of the fragments are highly felspathic trachytes, consisting of 

 small orthoclase laths with good flow-orientation, and containing phenocrysts of 

 dominant orthoclase and subordinate oligoclase. Occasionally small crystals of pale 

 brown augite are present. This rock is clearly the source of the broken crystals 

 described above. It varies in crystallinity ; some fragments have a cryptocrystalline 

 groundmass ; others consist of orthoclase phenocrysts in a pure yellowish-green glass 

 devoid of microlites. Many fragments consist solely of flow-orientated laths of ortho- 

 clase, and are free from phenocrysts, at least within the restricted limits of the 

 fragment. These are often of comparatively coarse grain, and may be either 

 porphyritic trachytes accidentally free from phenocrysts within the fragment, non- 

 porphyritic trachytes, or rocks allied to the bostonites. In some fragments the pro- 

 portion of oligoclase relatively to orthoclase increases until it equals or exceeds the 

 * Dixon and Vaughan, "The Carboniferous Succession in Gower," Quart. Journ. Geol. Soc, lxvii, 1911, p. 520. 



