276 FRANK D. ADAMS 



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-colored constituents, 

 it more closely resembles the latter. 



The large feldspars have frequently a peculiar crystalline 

 form giving to the mineral, when broken across, a perfect hexag- 

 onal 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 nephe- 

 line, 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 2.59 and 2.607, while that of the 

 other three was between 2.625 and 2.628. This shows the feld- 

 spar 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 inter- 

 growth of different species, described in the pulaskite. A par- 

 tial analysis of a specimen of this intermediate rock, from the 

 south side of the mountain, is given in the accompanying table 

 of analyses (No. VI) , on page 265. As will be seen, in chemical 

 composition as well as in mineralogical character, it occupies a 

 position intermediate between the essexite and the pulaskite, 

 occurring on either side of it, thus representing ah intermediate 

 zone in which the differentation was not quite completed. It is, 

 however, much more nearly allied to the essexite, being alkali- 

 calcic and dosodic, and although in the absence of a complete 

 analysis or detailed measurements its position in the new classi- 

 fication cannot be determined with absolute certainty, there is very 

 little doubt that it also, like the essexite adjacent to it, is an 

 andose. 



