REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



45 



Carlsbad law. We may point out, in speaking of these twins, that the plane of 

 union may vary in one and the same crystal. Fig. 6 shows a section of the 

 mineral twinned according to this law. It is cut almost parallel to the edge PjM; 

 and it may be observed that the plane of composition is now M, traces of which 

 appear on the two long sides of the section ; and again another plane, which may 

 correspond to a prism, perhaps to oo g 3, a face known to occur in sanidine. These 

 large sections have frequently an undulated extinction. The felspathic microliths of 

 the ground-mass are referable to the same species. The striae 

 of plagioclase are never observed. One form predominates — it 

 is that of extremely thin lamellae, which, when seen on the 

 face M, appear almost always twinned. Two of the individuals 

 are regularly superimposed, but one does not entirely cover the 

 other. Fig. 7 gives an example of one of these twinned 

 crystals : it shows two tabular individuals of sanidine super- 

 imposed on the face M, and twinned according to the Carlsbad 

 law ; traces of P and y are discernible, and the internal zones 

 give an indication of x. Extinction takes place with an angle 

 of + 5°, the angle P P' is 127° ; it is thus equal to that which 

 the same faces of sanidine twinned according to the Carlsbad 

 law form. The aspect of this twin may vary to infinity, but the fundamental form is 

 so constant that it is certain to occur in each preparation ; it is produced even when 

 the crystals become infinitesimal, as in the case of the very vitreous varieties of this 

 rock (see fig. 8). 



Fig. 7.— Trachyte of Red Hill. 

 Small twinned crystals of sani- 

 dine, two tabular individuals 

 superimposed on the face il/, the 

 traces of P y, and, in the internal 

 zones, the trace of x can he seen ; 

 the angle P P' is about 127°. h 

 crossed nicols. 



Flo. 8. — Trachyte of Red Hilh Large section of sanidine in a vitreous ground-mass is surrounded by small 

 lamellar crystals superimposed and twinned, embedded in the base. The cracks traversing the large section 

 are almost perpendicular to M. J s crossed nicols. 



Plagioclases, sharply distinguished by hemitropic striation, occur very rarely. Fels- 

 pathic sections may sometimes be observed showing some appearance of polysynthetic 

 lamellae, which are, however, indistinct compared with those of plagioclastic felspars. The 



