Bartrum. — Rocks of Mount Car gill, Dunedin. 167 



The Ground/mass. 



A network of predominant feldspar wraps around plentiful pale-green 

 aegirine - augite granules, a little fine nepheline, and a little iron -ore. 

 The feldspar, as a general rule, is in poorly shaped untwinned laths. 

 Fluxional arrangement is rare. There is a little polysynthetically 

 twinned plagioclase feldspar also present. The iron-ore is chiefly magne- 

 tite in small squares, but ilmenite is also present. 



The nepheline is only distinguished by staining the sections. It is in 

 minute hexagonal forms. 



No cossyrite was observed in the many sections prepared of this rock, 

 but there is an abundance of the mineral in a similar trachydolerite from 

 Mount Flagstaff. 



The granules of aegirine-augite are always irregular, and at times 

 simulate a mossv structure. Apatite forms stout though never plentiful 



P nsms - Order of Crystallization. 



Some of the relations are uncertain, but the probable order is (1) olivine ; 

 (2) apatite ; (3) amphibole ; (4) augite ; (5) sodalite ; (6) sanidine, nephe- 

 line, with possibly anorthoclase and oligoclase, and then, in the ground- 

 mass, iron-ore, aegirine-augite, nepheline, and feldspar, in the order named. 



Sections of a transition type of trachydolerite come from a little east 

 of the Main Peak. Olivine, in coarse, aggregates of fresh rounded crystals, 

 with a corrosion border of magnetite dust and aegirine-augite granules, 

 is very common. Pinkish augite has been corroded, and is edged by 

 aegirine-augite. Large crystals of resorbed amphibole are rare, but the 

 mineral may be represented by numerous small groups of secondary mag- 

 netic material. Feldspar sometimes encloses this magnetite. Nepheline 

 is rare. There is a little very opaque cossyrite. 



The groundmass is very dense and fine-grained ; it exhibits occasional 

 flow structure. Feldspar continues to be more important than the aegirine- 



Ohemical Characters. 

 Two analyses of the trachydolerite from two different localities are 

 appended, and with them, for purposes of comparison, two other analyses. 



Si0 2 



A1 2 3 



Fe 2 3 . . 



FeO 



MgO 



CuO 



K 2 



Na 2 . . 



H 2 



CI 



P 2 5 . . 



Ti0 2 



98-83 98-64 100-67 100-28 



A. Trachydolerite, Main Peak, Mount Cargill. (Analysis, J. Bartrum.) 



B. Trachydolerite, near Pine Hill. (Analysis, J. Bartrum.) 



C. Trachydolerite, Dr. P. Marshall.* 



D. Shoshonite, Yellowstone National Park.f 



* " Geology of Dunedin," Quart. Journ. (ieol. Soc, vol. 62, 1906, p. 407. 

 + Rosenbvpch, '' Elemente dev Gesteinslehre," p. 355, No. 13, 1901 ed. 



