96 PETROLOGY OF THE ALKALINE ROCKS 



vesicles are empty steam cavities devoid of drusy infillings. The less vesicular 

 varieties are dark greyish in colour, and have the same lustre. 



Included fragments of orthophyre and sanidinite (more acid differentiation products 

 of the same magma), of sandstone, and of older granulitic rocks occur in the Mount Cis 

 lavas. These are separately described by Mr. J. Allan Thomson. 



Composition. — The mineral composition of the trachytes of Mount Cis is almost 

 the same in all specimens examined. Felspar forms from 60 per cent, to 75 per cent. 

 of the mass, and segirine-augite from 10 per cent, to 25 per cent. The balance is made 

 up of olivine, magnetite, leucite, sodalite, interstitial nepheline (?), analcite, cossyrite, 

 riebeckite, and various ferritic decomposition products, some or all of which occur 

 together to the extent of from 5 per cent, to 10 per cent, in all specimens. Of these 

 magnetite usually forms up to 5 per cent., olivine up to 2 per cent. A cossyrite- like 

 hornblende occasionally replaces segirine so as to form upwards of 5 per cent. 

 Apatite is occasionally sparingly represented in long needles. Felspathoids range 

 from 1 per cent, to 10 per cent, in amount. 



Texture. — In describing textures the nomenclature proposed by Cross, Iddings, 

 Pirsson, and Washington is adhered to in most instances.* 



Most of these rocks have a vesicular structure, and are slightly porphyri tic- hiatal 

 with stray phenocrysts of the first generation, most of which are discernible to the 

 naked eye (phaneric). The base, which forms the main bulk of the rock, is 

 holocrystalline, or almost so, aphanitic, extremely fine-grained (microcrystalline to 

 cryptocrystalline), and moderately even-grained. The fabric is usually pilotaxitic or 

 trachytic. In the pilotaxitic specimens the fabric becomes trachytic near vesicles, 

 inclusions, and xenocrysts. In texture these rocks approach closely to that of the 

 phonolitic varieties of Warrumbungle Mountains trachyte | (New South Wales). The 

 textural characters of the whole of this group of lavas from Mount Cis may be briefly 

 expressed thus : holocrystalline or nearly so, megaporphyritic, perpatic, mediophyric. 



Inclusions. — The inclusions in the Mount Cis rocks consist of orthophyre, 

 sanidinites, pyroxene- granulites, diabase, and in some cases metamorphosed sandstone. 

 The first type is important as being autogenic, an earlier differentiation product of the 

 same magma ; it is similar to the Australian orthophyric trachytes in texture aud 

 composition, especially to those described by me from the Canoblas, New South Wales, 

 and the Glass House Mountains, Queensland. { They consist dominant ly of anortho- 

 clase with subordinate arfvedsonite, segirite, and aegirine-augite, and occasionally 

 a little olivine, which is not a common mineral in Australian orthophyres. It is 

 interesting in this connection to note that in Australian alkaline regions this lava 

 type was first erupted as plugs and mamelons, the more basic varieties (dark 

 trachytes, phonolitic and andesitic trachytes) being erupted later, largely from dykes 

 and fissures. It appears that in Ross Island no sanidinitic lavas have yet been found 

 except as inclusions. These, however, point to the same order of succession and 

 differentiation as in Australia. 



Minerals of the Mount Cis Rocks. — (a) The felspars are mostly of the acicular or 

 lath-formed habits, and consist of anorthoclase and sodasanidine. The sparing felspar 

 phenocrysts may have a stouter prismatic form or they may be tabular. They belong 

 to an earlier generation, the intratelluric period of consolidation, and usually exhibit 

 corrosion phenomena on the edges. They consist variously of anorthoclase, 

 sodasanidine, albite, and oligoclase, which are readily distinguished from one another 



* Jour, of Geo!., vol. xiv, No. 8, 1906. 



j Proc. Linn. Soc. ofN.S.W., 1907, vol. xxxii, p. 557. 



J Ibid., 1909, pt. i, vol. xxxiv, and 1906, pt. i, vol. xxxi. 



