238 Canadian Record of Science. 



This transitional rock is composed of the same 

 minerals as the essexite with the exception of the feldspar, 

 which consists in part of the soda-orthoclase characteristic 

 of the pulaskite, and in part of the plagioclase (in this 

 case oligoclase) which forms the feldspathic element of 

 the essexite. It is thus in mineralogical composition 

 intermediate between these two rocks, although, as above 

 mentioned, being rich in the dark-coloured constituents, it 

 more closely resembles the latter. 



The large feldspars have frequently a peculiar crystal- 

 line form giving to the mineral, when broken across, a 

 perfect hexagonal outline. The six faces represented in 

 this form are apparently T, L, and M. The crystals hold 

 many little inclusions of pyroxene, biotite, hornblende, 

 magnetite, sphene, and nepheline, often regularly arranged 

 so as to give a zonal structure to the feldspar individual. 

 The specific gravity of twelve small fragments of the 

 feldspar of these large crystals, collected from a locality 

 on the southern side of the mountain and as free as 

 possible from all inclusions, was determined. The specific 

 gravity of nine of these lay between 2. o 9* and 2.607, while 

 that of the other three was between 2.625 and 2.628. 

 This shows the feldspar in the former case to be identical 

 with that of the pulaskite, while in the latter three the 

 specific gravity lies between that of albite and oligoclase. 

 The somewhat greater specific gravity in this case may be 

 due in part to inclusions of other minerals. A separation 

 of the constituents of the rock shows, however, that, as 

 above mentioned, a considerable amount of oligoclase is 

 really present. The feldspar individuals, both great and 

 small, usually show in thin sections the mottled character 

 due to the intergrowth of different species, described in 

 the pulaskite. A partial analysis of a specimen of this 

 intermediate rock, from the south sicte of the mountain, is 

 given in the accompanying table of analyses (No. VI), on 

 page 220. As will be seen, in chemical composition as 

 well as in mineralogical character, it occupies a position 





