360 T. W. E. DAVID, F. B. GUTHRIE, AND W. G. WOOLNOUGH. 



in microstructure between the large and the small crystals ; (c) 

 there is a total absence of any trace of resorption ; (d) the larger 

 nephelines are surrounded by segirines more densely packed than 

 in other parts of the base, pointing to the fact that these nephelines 

 have grown in size in the place where they are now found, pushing 

 the segirines aside in the process. If we accept Rosenbusch's 

 definition for "porphyritic structure," that term cannot therefore 

 be applied to the rock under consideration. 



The most obvious and important mineral constituent is nepheline. 

 This occurs as occasional, fairly large, idiomorphic crystals in 

 stumpy hexagonal prisms with basal planes. These vary in size 

 up to 3 mm. in length, the breadth being about the same. Mr. 

 G. W. Card states that he has observed well developed pyramid 

 faces modifying the rectangles, but such faces appear to be excep- 

 tional. The mineral is clear and colourless and its refractive index 

 and double refraction are characteristic. Tn places, grey, yellow 

 and brownish decomposition products are abundant. The large 

 nepheline crystals have a marked zonal structure, the central 

 zones possess a higher refractive index than those near the 

 periphery, as proved by testing them by Becke's method. 



In the small crystals a similar arrangement can be made out 

 under the high power. The individualised inclusions are mainly 

 referable to segirine, similar to that occuring as an essential con- 

 stituent of the rock. It is recognisable by its marked and charac- 

 teristic pleochroism. Occasional grains of magnetite also occur. 

 Besides these, there are indeterminate dusty inclusions of a greyish 

 colour. In one very large crystal is included a fragment of the 

 felspathic material to be described later. Another large, almost 

 basal, section shows a very remarkable inclusion. The nepheline 

 is about 1 "58 mm. in diameter, in it is a crystal of sanidine, "53 mm. 

 long by *118 mm. broad. The sanidine is singly twinned, and has 

 given rise to cleavage cracks (parallel to [1010]) in the host 

 (Plate i, fig. 4). This occurrence is remarkable, as the crystallisation 

 of sanidine is usually subsequent to that of nepheline. The 

 essential felspar is present in small quantity, and is inconspicuous. 



