1!Y W. X. BEXSOK, \V. S. DUX, AND \V. R. CROWXE. 407 



4 



of eoutact metamorpbism. Augiilar aggregates eompused of tiuy clialcedonic 

 quartz-granules are numerous, and the rock is crossed with veinlets of the same 

 material, while many of the rock-fragments included in the tuff have been devitri- 

 tied. Tliese fragments include examples of a trachytic rock, a.s well as the por- 

 pliyritic rock already mentioned. This latter proves to he keratophyre, the only 

 phenocrysts recog-nisable microscopically being albite ; a few aggregates of chlorite 

 and quartz may possibly represent original pyroxene. In the larger fragments 

 the glassy base has been completely kaolinized and is characterised throughout by 

 well-marked perlitic cracking with traces of devitrification : in the smaller frag- 

 ments tlie base has comidetely dexitritied into spongy-looking fels]}athic material 

 with a lower R.I. than Canada Balsam. 



Underlying the andesite sill there is a tuff (1-155) of rather unusual constitu- 

 tion, consisting as it does for the most part of albite, pyroxene and fragments of 

 trachytic rock. The felspar is generally fragmental, but in some cases shows 

 almost perfect scjuare sections. It often contains inclusions of apatite needles 

 and of chloritic material, which at times assumes the rounded outline characteristic 

 of inclusions of glassy base. The pyroxene, which is mostly augite, with a little 

 hj'perstheue, still retains in a large measure its stout prismatic habit, although a 

 good deal cracked and somewhat altered . The rock fragments are small, none 

 being more than 3 mm. in diameter. Almost all of them have trachytic faliric 

 and are porphyritic in columnar albite. A few are partly gla.ssy. The inter- 

 stices of the rock are filled mostly with cidorite. but some parts of the matrix 

 appear to consist of an extremely fine-grained trachytic hypocrystalline rock, and 

 here and there are evidences of silicification . Tlie rock may be termed a pyroxene- 

 bearing keratophyre tuff. 



In view of the fact that the albite of many l)asic rocks is secondary and that 

 many keratophyres must be regarded as albitized porphyrites (49), it is i)roper 

 to in(|uire into the possibility that the albite in the rocks under discussion may be 

 secondary. As far as can be observed there is no positive evidence that all)itiza- 

 tion has occun-ed. Neither are there traces of still unaltered basic felspar (50), 

 nor does the albite exhibit a suspicious cleai-ness (51), nor yet is it crowded with 

 epidote and other alteration products, such as (me might expect in the endogenic 

 albitization of a basic felspar. The allute then may fairly be considered a primary 

 mineral . 



Felspathic Tuffs <,f tlie Kuttiuicf Series. 



These form an extensive series of gritty-looking rocks constituting much of 

 the Kuttung Beds and intercalated with the conglomerates. Megascopically, a 

 typical specimen obtained from the Main Felspathic Grit is iiinkish-grey in colour, 

 composed mostly of small felspar chips and crystals, both pink and white, and sub- 

 ordinate i|uartz. with a few dark minerals, all endx'dded in a greyish matrix. In 

 thin section the tutt'aceous character of the rock becomes apparent. Tlie dominant 

 mineral is orthoclase, in crystals and fragments averaging about 1 mm. and much 

 kaolinized; there is a minor amount of plagioclase, mostly albite. Quartz, of the 

 type found as phenocrysts in volcanic rocks, is fairly aljundant, mostly a.s frag- 

 ments but often retaining traces of crystal form and showing corrosion and stony 

 inclusions. A few small pieces of augite, hornblende and ilmenite are also seen. 

 Of lava fragments there are many, mostly glassy and often pumiceous, sometimes 

 possessing fluidal or spherulitic fabric. 



