371 



by the occurrence of large (often three to four centimetres long) 

 porphyritic crystals of a dark grey felspar in a violet-grey, fine-grained 

 ground-mass. The felspars frequently give rhombic sections in con- 

 sequence of their peculiar form ; hence the name rhomben-porphyry given 

 to the rock by L. VON BUCH. The ground-mass of the rock is formed 

 of felspar, augite (slightly pleochroic in violet, brown and green tints), 

 magnesia-mica, olivine, apatite ^often in large needles) and magnetite. 

 The altered rocks assume red tints. 



A good deal of discussion has taken place as to the nature of 

 the felspars in this rock. The porphyritic felspars are characterized 

 by the fact that the dominant forms are T, 1 and y. The angle 

 between the cleavages is approximately a right angle ; at least it 

 does not differ from this more than the cleavages in microcline. 

 The chemical composition is difficult to determine in consequence 

 of the occurrence of inclusions. It is certain, however, that in 

 addition to potash there is a considerable amount of soda and lime. 

 Under the microscope an extremely fine twinning is usually but not 

 always recognizable. Sections parallel to P (001) extinguish parallel 

 to the trace of M (010) when they do not show twinning and at 

 very small angles (1 to 2 C ) when twinning is present ; sections 

 parallel to M do not show twinning and the extinction angle is 

 here 5|-6|. Sections at right angles to P and M show extremely 

 fine twinning and sometimes possess the structure of microcline. 

 The felspar is termed soda-microcline by BROGGER. It belongs to 

 the anorthoclase group of ROSENBUSCH. The felspars of the ground- 

 mass are referred to orthoclase by ROSENBUSCH. They are for the 

 most part devoid of twinning. This rock is interesting to British 

 geologists because boulders of it occur in the Eastern Counties. 



The name pantellarite has been introduced by FOERSTNER^ for 

 certain lavas of the island of Pantellaria. The rocks are rich in iron 

 and alkalies. They consist essentially of anorthoclase, cossyrite (a 

 triclinic hornblende-like mineral rich in iron and soda), augite and 

 glass. 



The following analyses will give an idea of the composition of 

 keratophyre, rhomben-porphyry and pantellarite. 



(1) Z.K. (1883). VIII. 



