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frequently referred to as kaolinization, and it seems probable that, much of 

 the granular matter may be closely allied to kaolin in composition. The 

 decomposition products after orthoclase are often coloured red by ferric 

 oxide. Sometimes they are distributed irregularly through the mineral* 

 at other times the mica flakes have been more especially developed along 

 the cleavage planes. Dr. SOBBY has pointed out that the ground-mass 

 of some of the Cornish elvans (quartz-felsites) is mainly composed of 

 quartz and a micaceous mineral. It seems probable that this mica is a 

 secondary product developed at the expense of the original orthoclase. 

 Epidote granules are not unfrequently developed in connection with the 

 alteration of orthoclase. Under exceptional circumstances orthoclase may 

 be wholly or partially replaced by one or more of the following minerals 

 quartz, tourmaline, cassiterite, topaz. 



Under the influence of dynamic metamorphism, orthoclase is sometimes 

 replaced by a fine-grained mosaic of differently orientated individuals ; at 

 other times, and especially when a shearing movement takes place along 

 definite planes, it gives rise to white mica which may take the form either 

 of somewhat irregular crystalline scales or of a fine-grained aggregate of 

 irresolvable individuals. The felspar-mosaic is often developed round the 

 margins of the larger individuals, portions of which may remain as more or 

 less porphyritic elements in a ground-mass of granulitic texture (inortar- 

 structuro, mortel-structur, TOENEBOHM). The effects of dynamic metamor- 

 phism are best studied in rocks of granitic texture. When the felspar is 

 replaced by a mosaic the quartz is affected in the same way so that the 

 granulitic mass is usually an aggregate of quartz and felspar. Sometimes 

 the individuals of the secondary aggregate of quartz and felspar are so 

 minute as to be incapable of definite recognition. The aggregate must then 

 be described as crypto-crystalline. 



Microcline. This mineral is common amongst the granites with two 

 micas, rare in the biotite- and hornblende-granites, and almost entirely 

 absent from the trachytic rocks of acid composition. Where it occurs as a 

 normal constituent of the rock it is always without crystalline outline and 

 is stated by ROSEKBUSCH to be generally of later date than the orthoclase 

 in the same rock. It is, however, frequently found intergrown with this 

 mineral. Microcline with crystalline outline is found in granites with drusy 

 cavities and the forms are analogous with those of orthoclase. The most 

 striking feature of microcline is its polysynthetic structure. Cleavage 

 flakes or sections parallel to P show this structure in great perfection. The 

 mineral is seen to be made up of narrow and more or less spindle-shaped 

 lamella,' which intersect approximately at right angles and thus produce the 

 well known cross-hatching. The lamellae parallel to the edge P: M in 

 flakes parallel to P extinguish at angles of 15 on either side of the line of 

 separation. They correspond to twinning on the albite plan. The second 

 set of lamellae is generally regarded as due to twinning on the pericline 

 plan. The lamella of microcline are rarely so sharp and well defined as 

 those of the other triclinic felspars and there is almost always a tendency 

 to a spindle-shaped form in sections at right angles to tlie planes of lamella- 



