170 Transactions. 



Thus field relations give no help in drawing distinctions between different 

 petrological types, but indicate that the origin of all is differentiation of 

 the one magma. There is. however, a possible exception to this, for an 

 outcrop of nephelinitoid phonolite on Butter's Peaks may be a dyke. 



Under the microscope the chief feature of the type is the nephelinitoid. 

 or cellular, structure of the groundmass, due to the numerous minute 

 hexagons of nepheline seen under moderate magnification. 



The phenocrysts are typically allotriomorphic ; the most common are 

 pinkish-brown augite, sodalite, sanidine, nepheline, and olivine. 



The nepheline is large and well-shaped, but is crowded with minute 

 liquid inclusions. The crystals of sodalite are very large, and are usually 

 crowded with minute gaseous inclusions ; they show good dodecahedral 

 cleavage. 



Sanidine is clear and glassy, but exhibits shadow extinction. A few 

 characteristic anorthoclase phenocrysts occur. 



Olivine has either a dense corona of aegirine-augite with magnetite, or 

 else a corrosion fringe of aegirine-augite and magnetite dust. 



Pleochroic aegirine-augite is shown in a few well-shaped crystals that 

 have suffered slight resorption, and have been edged by the common 

 pinkish augite. This latter variety sometimes encloses resorbed amphibole, 

 showing that it did not separate out till after, or was connected with, the 

 resorption of the amphibole. 



The groundmass is holocrystalline but fine-grained, and generally similar 

 in minerals to that of the other types of trachydolerite. 



Aegirine-augite in mossy granules is dominant ; if often assumes a 

 lath shape, and then shows more or less parallel alignment. 



In sections of those rocks that, both petrographically and in field 

 occurrence, approach the nephelinitoid phonolites cossyrite occurs plenti- 

 fully, but is absent in other sections, unless some minute opaque dentritic 

 growths can be referred to this mineral. 



Feldspar is moderately important, and enwrapping laths show up 

 amongst the numerous minute hexagonal forms of nepheline. Very little 

 magnetite is present ; there are a few scattered flakes of sodalite. 



No analysis of this rock was made. 



B. Nephelinitoid Phonolite. 



As would be expected, in certain places this rock merges closely into 

 the type of trachydolerite just described. In several sections segregations 

 or inclusions of the basic trachydolerite previously described are very 

 typical. They average about 7 mm. in diameter, and are most probably 

 of the nature of segregations. 



Leucite again appears as a subsidiary mineral. It is difficult to dis- 

 tinguish it from numerous other rounded isotropic forms that are judged 

 from their ready gelatinization with dilute acid, and from the high percent- 

 age of chlorine in the rock, to be sodalite. The leucite is in small rounded 

 or idiomorphic shapes, and commonly shows characteristic radial arrange- 

 ment of included aegirine-augite granules. 



In hand-specimen this rock is very similar to the dense basic variety 

 of trachydolerite, but has a somewhat lighter colour and greasier appearance. 

 It weathers very readily. 



Under the microscope true phenocrysts are rarely seen, unless in the 

 proximity of the basic inclusions, where pink augite and olivine especially 

 are common. 



