259 



occurring at CanispW in Sutherlandshire may be regarded as a typical 

 oligoclase-biotite-porphyrite. 



The mica-traps must not be confused with the normal mica-porphy- 

 rites. They constitute a portion of a somewhat exceptional group of rocks 

 the lamprophyr-group of GUMBEL and ROSENBUSCH and will be 

 described in another chapter. 



CHARACTERS OF THE ROCK-FORMING MINERALS. 



Felspar. Two generations of felspar are almost always recognizable 

 in the andesites ; in the granular diorites only one generation is present 

 as a rule. The felspars of the first generation in andesites are generally 

 either labradorite, andesine or oligoclase ; bytownite and even anorthite 

 have, however, been recorded. They are frequently somewhat tabular in 

 form with conspicuous development of the brachypinacoid. Glass inclu- 

 sions and inclusions of the ground-mass are very common. These inclusions 

 may be scattered irregularly through the crystal; more frequently they 

 occur in greater abundance either in the centre or near the margin ; and 

 not seldom they are found to be arranged in a number of concentric zones. 

 Inclusions of the ferro-magnesian constituents and iron-ores also occur in 

 the felspars. A zonal structure due to a variation in the optical proper- 

 ties of successive layers is by no means uncommon. Very often the 

 felspars have been broken by interstitial movements which have taken 

 place in the magma subsequent to their development. Twinning on the 

 albite plan may almost always be observed and very frequently this is asso- 

 ciated with twinning on the Carlsbad and pericline plans. 



The felspars of the ground-mass occur in small columnar forms giving 

 lath-shaped sections. The direction of elongation is that of the edge 

 010:001. These felspars sink to the smallest dimensions (microlites) and 

 are often so numerous as to produce a kind of felt-like aggregate the 

 " mikrolithenfilz " of German authors. In composition they appear to be 

 always more acid than those which occur as porphyritic constituents. 

 Thus, M. FOUQUE has shown that in a pyroxene-andesite from Santorin, the 

 porphyritic felspars are labradorite and the microlites of the ground-mass 

 albite. Microlites giving nearly straight extinction in all longitudinal 

 sections (oligoclase) are common in many andesites. The felt-like aggregate 

 of felspar microlites may, in a certain sense, be regarded as the charac- 

 teristic feature of the andesites. It is very rarely observed in the volcanic 

 rocks of basic or acid composition. 



In the plutonic rocks of intermediate composition the plagioclase 

 belongs as a rule to only one period of consolidation and the crystalline 

 form is rarely so well shown as in the porphyritic crystals of the volcanic 

 rocks. Liquid and gas inclusions are present, but glass inclusions are 

 comparatively rare. Orthoclase is frequently present and by an increase in 

 the amount of this constituent the diorites shade so gradually into the 

 granites that it is often impossible to say to which class a given rock 



(1) Some portions of the Canisp mass contain orthoclase, often zoned with oligclase. 



