BY W. G. WOOLNOUGH. 537 



and under the microscope felspars of the second generation are 

 small and not plentiful. On the other hand, ferromagnesian 

 minerals are very abundant. 



The base is certainly hyalopilitic, the amount of isotropic 

 material being variable but notable. Under the magnification 

 necessary to examine it, it is only very faintly yellow, and 

 not much darker under low powers. It is crowded with 

 hair-like crystallites, for the most part isotropic. The microlitic 

 components of the base include plagioclase and augite, with a 

 marked fluxion arrangement. Both minerals are of minute 

 dimensions. Augite is the more plentiful of the two, and is very 

 similar to that already described in other rocks. It is perhaps 

 more strongly refracting, and its double refraction is greater. 

 The felspar is mostly striated after the albite law. The refractive 

 index is greater than that of Canada balsam, where a difference can 

 be detected. Fairly good symmetrical extinctions up to 30° can 

 be measured; with a difference of over 10° in the halves of an 

 occasional Carlsbad twin, this indicates labradorite. 



Of the phenocrysts, augite and olivine are much the most 

 abundant. The former occurs in large sections, up to as much 

 as 6 mm. diameter. It is beautifully idiomorphic, the forms 

 indicated by the outlines of the sections being ■{ 100 )>, -{ 010 J- , 

 ^ 110 |>, ^ 111 J> and -{ 001 |.. The length is on the whole 

 not much greater than the breadth, so that the sections are more 

 or less equidimensional. The colour is rather variable ; in 

 different sections it shows tints of greenish-yellow or olive-green 

 considerably different from one another. Notwithstanding this, no 

 pleochroism is noticeable in any one section. Even in one and the 

 same cr3'stal the colour varies zonally, showing that the difference 

 in colour depends on variation in composition and not on the 

 direction of the section, thus explaining the absence of pleochroism. 

 Some of the sections show^ most beautifully the " hour-glass " 

 structure often met with in augite. Cleavage, refractive index 

 and double refraction are quite usual. Twinning after the 

 orthopinacoid law is not very common, though beautiful examples 



