COLLECTED AT CAPE ROYDS 173 



of the vertical axis is parallel to the plane of vibration of the polarizer, it becomes 

 opaque. 



A few small but sharply defined crystals of oligoclase are included in the anortho- 

 clase. 



A noteworthy feature is the very considerable abundance of sharply prismatic 

 apatite. 



This rock is a most peculiar type. In mineral composition it is like laurdalite, 

 except that sodalite is substituted for nepheline. There does not appear to be any 

 alteration product present which would have been formed from nepheline. 



443. Quartz-diorite (Plate I, Fig. 4) 



Macroscopic Character s. — A most strikingly handsome rock type. It is fairly 

 coarsely crystalline and is mottled black and white. Sharply idiomorphic prisms 

 of lustrous hornblende up to 10 mm. by 2 mm. are scattered very abundantly through 

 a base of the lighter coloured constituents. Less abundant but quite as conspicuous 

 are very large plates of biotite 18 mm. in diameter, more or less chloritised. Yellowish 

 orthoclase occurs in pseudo-porphyritic grains up to 20 mm. diameter, containing 

 very abundant inclusions of hornblende, biotite, and idiomorphic, colourless 

 plagioclase. 



In the light-coloured interstitial material orthoclase, plagioclase, and not very 

 abundant quartz can be recognised. 



A very remarkable feature of the rock is the habit and abundance of apatite, which 

 occurs in yellowish and colourless needles up to 8 mm. long by 0*2 mm. thick. 



Microscopic Characters. — The texture of the rock is holocrystalline 

 porphyritic with hypidiomorphic granular base of medium grain. 



Under the microscope the most conspicuous and abundant minerals are the felspars, 

 of which there is a considerable variety. 



Orthoclase composes the largest grains in the rock. In addition, there are smaller 

 grains and patches of the same mineral scattered through the slide. The grains are 

 all completely allotriomorphic, the larger ones poikilitically enclosing crystals of most 

 of the other minerals. Towards quartz the mineral shows occasional traces of crystalline 

 outline, but often the two minerals are intergrown. It is slightly clouded by decom- 

 position, and shows veins due to perthitic intergrowth. All the sections are 

 untwinned. 



Plagioclase exists in at least two habits, corresponding to two different varieties : 

 (a) strongly idiomorphic thick tabular crystals, twinned after the albite and pericline 

 laws, and often after the Carlsbad law as well ; (b) in subidiomorphic to allotriomorphic 

 grains with Carlsbad and albite twinning. In the former case there are strongly 

 marked zonal effects which are perfectly continuous from periphery to centre. 



Those of the first set are frequently included in individuals of the other type. In 

 the central portions of the former the symmetrical extinctions of the albite lamellae 

 give a maximum of 32°, with a difference (A) of 21° for the extinction in the other 

 half of the Carlsbad twin. The peripheral portions give perfectly straight extinctions. 

 Hence the variation is from Ab 4 Anj on the outside to Ab g An n at the centre. 



The felspars of the second period are less strongly zoned than those of the first, 

 though still quite a noticeable "undulation" in the extinction occurs. The angles 

 measured point to Ab 3 Anj as the composition of the interior while the periphery is 

 more acid. 



All these felspars are irregularly clouded by decomposition products (chiefly very 

 fine grains of calcite) and are dusted round their edges with similar material. This 



