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y (201). When present in the form of irregular grains the unstriated 

 felspars frequently constitute a kind of ground-mass in which the other 

 constituents lie embedded. The striated felspars may occur in forms giving 

 long lath-shaped sections, or short and broad sections in which the lath- 

 shaped form is not so strongly marked. Where both striated and unstriated 

 forms occur the former are usually idiomorphic with respect to the latter. 

 In the kersantites of Brittany unstriated felspar and quartz are frequently 

 intergrown so as to form micro-pegmatite. The bulk analyses of the 

 rocks and micro-chemical reactions on the minerals themselves show 

 that the felspars must be somewhat abnormal in composition. In many 

 cases they probably belong to the anorthoclase group of ROSENBUSCH. As 

 already stated the felspars have frequently lost their original characters 

 by alteration, and a precise classification of the rocks is thus often 

 rendered impossible. Cloudy granular matter, usually coloured red by 

 iron-oxides, has been produced and the mass then gives only aggregate 

 polarization. Calcite is also richly developed by the processes of 

 decomposition. 



Augite. When fresh this mineral is almost always nearly colourless 

 or a very pale green. It resembles the augite (malacolite) of the augite- 

 granites and of the basic diorites of Warwickshire and Inchnadampf. 

 According to ROSENBUSCH it frequently contains octahedra of chromite 

 or picotite. By alteration it gives rise to chloritic or serpentinous sub- 

 stances. Sometimes it is changed to uralite. 



Hornblende. This mineral may occur, according to ROSENBUSCH, 

 in short columns or in long prisms. It may be brown or green in 

 colour. A blue variety (not glaucophane) has been observed in a 

 minette from Wachenback in the Vosges. Zonal structure and twinning 

 are common. The crystalline forms are those usually found in the rock 

 forming hornblendes. Basaltic hornblende occurs in the camptonites 

 of ROSENBUSCH. Alteration develops chloritic and serpentinous substances 

 with which carbonates are frequently associated. 



Accessory minerals. Apatite plays a very important part as 

 an accessory constituent. It is frequently present in extraordinary 

 abundance (e.g., augite-minette of Trelissick Creek, Cornwall), and it 

 is often the only mineral which remains unaltered. Carbonates have 

 already been referred to. They may occur in the form of crystalline 

 powder irregularly diffused through the mass of the rock or as definite 

 crystalline plates. 



Olivine has frequently been observed in Continental minettes and 

 kersantites. BECKE proposed the name of pilite-kersantite for a rock 

 in which the pseudomorphs of olivine contained innumerable tremolite 

 needles, somewhat similar to those which occur in the olivine-pseudomorphs 

 of the Menheniot picrite. Iron-ores are present in almost all the rocks. 

 Magnetite, ilmenite and hematite have been observed. Quartz is frequently 

 present and appears to be both an original and a secondary constituent. 

 The original quartz may occur as grains or as a constituent of micro- 

 pegmatite. Secondary quartz occurs in irregular grams. 



