1 903 J Petrography of some Igneous Rocks. 8q 



origin, a fine-grained dark diabasic rock, inclined in places to be 

 porphyritic in structure." 



The collection contains two specimens from this locality, re- 

 presenting two different varieties ot the same rock, one showing a 

 massive and porphyritic character, while the other is scoriaceous. 



Under the microscope, the rock is seen to consist of a micro- 

 crystalline groundmass composed of minute laths of felspar, show- 

 ing a trachytic structure and often a fluidal arrangement. In the 

 groundmass are included well defined phenocrysts of plagioclase, 

 augite and biotite, also a few phenocrysts composed of a micro- 

 perthitic intergrowth of two felspars, one of which may be ortho- 

 clase. Some of the plagioclase individuals are twined according 

 to both the Carlsbad and Albite laws, which when examined 

 by Michel Ldvy's method were found to have the composition 



Abj Anj. 



In the second section, which is a highly vesicular variety, the 



vesicles nearly all show a narrow border of what appear to be 

 zeolites which are just beginning to form in the cavities. The 

 biotite, in this section, is dark brown in colour and contains little 

 black needles arranged in skeleton patterns, which probably con- 

 sist of rutile derived from the partial decomposition of the mica. 

 The groundmass of this section is relatively more abundant than 

 the vesicles and it is therefore not a true pumice, though it ap- 

 proaches one in appearance. 



The rock is very fresh ; and, from a comparison with the 



other rocks described, it would appear that this is a comparatively 

 recent lava flow. It has the character and mineralogical composi- 

 tion of an andesite. 



No. 17. — " Reddish-grey rock from the north side of Slate Creek, 

 near its mouth." 



Highly decomposed basic igneous rock, having the composi- 

 tion of an augite andesite. 



Under the microscope, the rock is seen to possess a well 

 marked porphyiitic structure. The groundmass is micro- 

 crystalline, consisting of minute laths of felspar, which often 

 show a fluidal arrangement, and a little chlorite and oxide of 

 iron, representing alteration products of some ferromagnesian 

 constituent. Through this are distributed large well defined lath- 



