502 I. H. OGILVIE 



The rock is probably to be referred to the group described 

 by Professor Pirsson 1 as "the analcite group of igneous rocks," 

 and no similar rock has hitherto been described from this region. 

 Similar masses are mentioned in the Elmoro folio, and termed 

 "lamprophyres," hence it is probable that future investigation 

 will bring to light other related intrusions.. No elaeolite syenite 

 nor other rock that could be related to these lamphrophyric 

 intrusions has yet been found ; hence the relationship of the rock 

 and the magmatic history of the group to which it belongs, 

 remain as problems to be studied in connection with the further 

 investigation of the region. 



The rock is medium grained, of a general gray appearance, 

 with large crystals of hornblende and augite which can readily 

 be seen with the naked eye. In the predominating phase of the 

 rock, these crystals are about .8 mm in length, but in occa- 

 sional segregations they reach a length of as much as I cm . 

 These ferro-magnesian minerals lie in a gray groundmass 

 which on microscopical examination proves to be mostly plagio- 

 clase. 



In thin section the rock is seen to be porphyritic, with phen- 

 ocrysts of augite and of hornblende, occurring in equal 

 amounts, and of rarer biotite. The groundmass consists of a 

 network of plagioclase, with an isotropic substance which is 

 probably analcite, filling the interstices between the plagioclase 

 laths. Magnetite, ilmenite, and apatite are also present. 



The pyroxene is always idiomorphic, occurring in large phen- 

 ocrysts and also to a small degree in the groundmass. It is a 

 pale greenish-violet, normal augite, and is very faintly pleochroic. 

 The cleavage is well defined. Some of the porphyritic crystals 

 show a slight zonal structure. Twinning is very common and 



usually the twinning plane is ooP— (100). Certain complicated 

 intergrowths also occur, which probably also represent twinning, 

 the twinning planes being — P.L (ioi) and P 2 (122) (Fig. 2). 

 1 L. V. Pirsson, Jour. Geol., Vol. IV, 1896, pp. 679-690. 



