MINERALOGY AND PETROLOGY. 941 



Orthoelase.] 



is amenable to general chemical transformation, a great change takes place. The 

 crystal becomes larger by the absorption of a portion of the surrounding matrix, 

 which is also finely orthoclastic, and a rim of fresh, perfectly transparent orthoclase 

 is formed all around the original grain. Such new matter also penetrates within the 

 old crystal along its fissures, recementing the separated portions by fine lines of new 

 orthoclase. It also creeps in between the cleavages, especially if they are open enough 

 to serve as solution planes, and by depositing a cement of fresh material it regenerates 

 the old crystal. At the same time while this general enlargement and reconstruction is 

 going on, the muscovite scales and other alteration products tend toward the centre 

 of the old grain. It is not impossible that this apparent greater centralization of 

 these impurities in regenerated grains may be due wholly to the enlargement of the 

 borders and the general restoration of the chemical integrity about the peripheries 

 of the old grain, and that hence the exact locus of the individual muscovite scales in 

 the body 01 the crystal is not changed by the regeneration; but in many cases, it 

 certainly appears to be the fact that such impurities are crowded closer together 

 and generally toward the centre. Such seems to be the case, at any rate, when in 

 the same grain the impurities are gathered in two or three places by reason of some 

 variation in the manner of influx of the fresh matter, instead of remaining, as at 

 first, uniformly scattered throughout the crystal. These features of regeneration are 

 seen in nearly all granites and gneisses. (See Nos. 1278, 1427, 1436, 1728, 1980, 1992, 

 2229, 2276.) 



In general, the new orthoclastic material is perfectly oriented with the old. 

 Indeed, it is probable that, as with quai"tz, the whole of the old crystal is revamped and 

 undergoes a complete recrystallization, a change which would of course allow if not 

 require the concentration of the inclusions at one or two places, in groups. But it 

 has been noted, in a few instances, that the new feldspathic material is not oriented 

 with the old. One instance (No. 1051) was carefully examined and it was found 

 that the new feldspar had the optic characters of a "deformed orthoclase," i. e., that 

 the positions of the elasticity axes n f and n m were the reverse of those in the original 

 grain. In several cases (illustrated by rock No. 1515, and plate V, figure 7) it also 

 appears that this manner of interpenetration by new orthoclase presented a rather 

 spreading microclinic arrangement, and such arrangement only appeared in certain 

 areas in the old grain (Nos. 1992, 2194). The idea that the microline structure is 

 wholly caused by such new intercleavage growths in an old feldspar was rather 

 strengthened when it was observed that the microcline structure itself is uniformly 

 most prevalent in such regenerated rocks (granites, etc)., and that its crystals some- 

 times grade off into non-microclinic orthoclase. 



This "altered" condition of the orthoclases in granites, etc., has sometimes 



