352 



such rocks minette-felsite ; when the dominant felspar is plagioclase he 

 uses the term kersantite-porphyrite. The determination of the presence of 

 interstitial matter is rendered difficult by the fact that the felspars of the 

 ground-mass have frequently lost all individual action on polarized light. 



Mica-diabase. This term should be applied to altered plagioclase- 

 augite rocks rich in brown mica. The augitic camptonitcs of RosENBUSCH, 

 in which mica plays an important part, might be termed mica-diabase. 



Mica-diorites. This term might be used with appropriateness for 

 the hornblendic camptonites of Professor ROSENBUSCH in which mica 

 plays an important part. 



Mica-syenite. This term is the equivalent of minette. 



Minette-felsite, Kersantite-porphyrite. The sense in which these 

 terms are used by Professor BONNEY has already been explained. 



Mica-porphyrite. This term should only be applied to rocks in 

 which felspar occurs as a porphyritic constituent. The typical mica- 

 porphyrites belong to the intermediate group ; not to the group under 

 consideration. 



CHARACTERS OF THE ROCK-FORMING MINERALS. 



Mica. This is the most important and characteristic mineral. It 

 occurs in thin plates which are often extremely ragged at their edges. 

 They frequently contain intrusions and inclusions (apparently) of the 

 ground-mass. In colour they are a rich, but somewhat pale brown. 

 Sections parallel to the vertical axis are dichroic but not so markedly 

 so as in the mica of the granitites and quartz-diorites. When viewed 

 with rays vibrating at right angles to the cleavage cracks such sections 

 appear colourless in thin sections. Under crossed nicols they give vivid 

 polarization tints, which, however, do not appear to be so high as those 

 of muscovite. The central portions of the crystals are often lighter in 

 colour than the marginal portions. They are biaxial with a small optic 

 axial angle. The optic axial plane is sometimes parallel to two of the 

 edges of the hexagon ; sometimes at right angles to these edges. In 

 other words the mineral sometimes belongs to the meroxene and lepi- 

 domelane group of micas, sometimes to the anomite group. The latter 

 case was observed by Mr. DICK in the mica of the Roseash rock described 

 by Mr. DOWNES. (I) The angle for blue rays was found to be about 7 in 

 air, that for red rays was about 16. The dispersion, therefore, was p > v. 



Alteration sometimes appears to bleach the mica; at other times it 

 converts it into chlorite. Calcite and epidote are frequently found in 

 lenticles between the lamellaB of the more or less altered micas. This 

 mica often shows lamellar twinning in sections at right angles to the 

 principal cleavage. 



Felspar. The unstriated felspars occur in more or less idiomorphic 

 crystals and irregular grains. In the former case they give lath-shaped 

 sections which are usually only two or three times longer than broad. 

 The dominant forms, according to ROSENBUSCH, are P (001), M (010) arid 



(1) Trans. Devon. Ass., Vol. XVI. (1884), p. 498-500. 



