256 



In chemical composition it is intermediate between andesine and labra- 

 dorite. Hypersthene occurs in brownish-red grains and crystals. The 

 pleochroism and other optical characters may be determined in those 

 sections which show crystalline form. The enstatite is distinguished from 

 the hypersthene by the absence of colour. Both minerals frequently 

 show the characteristic alteration to bastite. Diallage and augite 

 occur in irregular grains. These minerals are often intergrown with 

 hypersthene and biotite. Biotite occurs in irregular plates of reddish 

 brown colour, which show a most intense pleochroism. It changes 

 into a green chloritic mineral throughout which yellow epidote grains are 

 usually scattered. Quartz is almost if not always present, but varies 

 in amount in different specimens. It occurs in grains and granular 

 aggregates, but never shows good crystalline outline. It contains fluid 

 inclusions and occasionally grains of augite and small plates of biotite. 

 It is sometimes intergrown with felspar in the form of micro-pegmatite 

 and in some of the more acid rocks the micro-pegmatite forms a kind of 

 ground-mass. Magnetite occurs in grains and is often attached to biotite 

 in which it always occurs as inclusions. Apatite is especially abundant 

 in the rocks which contain much quartz. It occurs in the form of very 

 long thin needles. The authors separate the rocks into four principal 

 groups which, however, are connected by intermediate forms. They pro- 

 pose the following names for the four groups : (1) Norite, (2) Quartz-norite, 

 (3) Norite-porphyrite, (4) Quartz-mica-diorite. These groups are denned 

 by the relative proportions of the different constituents and by the presence 

 or absence of porphyritic constituents. Quartz is present in all of them, 

 but is less abundant in the norites than in the other members of the group. 

 It will be observed that the presence of quartz in the norites of TELLER and 

 VON JOHN separates these rocks from the typical basic norites. In fact 

 these quartz- bearing norites bear the same relation to ordinary norites 

 that the quartz-gabbros of Carrock Fell do to ordinary gabbros. The 

 norites and quartz-norites occasionally become porphyritic by the con- 

 spicuous development of plagioclase and a rhombic pyroxene. Orthoclase 

 may sometimes be observed in porphyritic crystals. By an increase in 

 the amount of biotite and a decrease in the amount of pyroxene the 

 rocks of the norite- group pass into pyroxene-bearing quartz-mica-diorites 

 and finally into typical quartz-mica-diorites. These are the most acid 

 members of the group. 



The following analyses illustrate the composition of the group : 



