29S 



in the trachytic rocks. In the latter they usually take the form of zoning 

 and it is an interesting fact that no general rule can be laid down as to the 

 order of development of the two species. Sometimes orthoclase is found 

 surrounding plagioclase (Mont Dore, trachyte), at other times plagioclase is 

 found surrounding orthoclase (porphyrite, Canisp). In all such cases the 

 plagioclase appears to be oligoclase or oligoclase-albite. 



In the granitic rocks and in the gneissose rocks of corresponding 

 composition the intergrowths take the form of perthite and micro -perthite. 

 Sections of orthoclase then exhibit a fibrous structure. Those parallel to 

 M show a number of narrow and somewhat irregular strips or patches 

 arranged with their longest axes parallel with the vertical axis of the crystal. 

 These patches extinguish at a different angle to the main mass of fche 

 crystal and often possess lamellar twinning. Sections parallel to P show 

 a less regular arrangement in the interpositions. The plagioclase appears 

 in all cases to be either albite or oligoclase. The intergrowths of the 

 above type may be observed on all scales of magnitude from macros- 

 copic perthite down to the finest micro-perthite. It probably also occurs 

 on an ultra-microscopic scale. LEHMANN regards rnicro-perthite as 

 a secondary structure arising in connection with the metamorphosis of 

 soda-orthoclase. 



The specific gravity of orthoclase is less than that of any other felspar 

 (2*54-2'59). The typical orthoclase-molecule is represented by the formula 

 K,0, A1 2 3 6Si0. 2 Analyses invariably show a certain amount of 

 JSla 2 0. This Na 2 O is present in a molecule of the albite type. The 

 substance analysed may consist of a macroscopic, microscopic or ultra- 

 microscopic intergrowth of typical orthoclase and typical albite ; or of 

 typical orthoclase and some soda-lime felspar. 



The orthoclase of the second generation in porphyritic rocks may be 

 idiomorphic or allotriomorphic. In the former case it rarely occurs in forms 

 which give elongated lath-shaped sections. Square or nearly square 

 sections bounded by traces of the faces 1', M and y, are the most common. 

 In holocrystalline rocks the relation of the quartz to the orthoclase is a 

 matter of considerable importance. The experience of the author does not 

 bear out the view that any rigid rule can be laid down to express the order 

 of crystallization of these two minerals. Sometimes the orthoclase 

 precedes the quartz, sometimes the two minerals crystallize simultaneously 

 (granophyres), and sometimes the orthoclase plays the role of ground-mass 

 to quartz grains which show more or less definite boundaries. 



In the non-porphyritic granites it is generally impossible to recognize 

 two generations of orthoclase. In these rocks the orthoclase rarely shows 

 good crystalline form. It has crystallized approximately at the same time 

 as the quartz and, where micro-pegmatitic structures have not been 

 developed, the two minerals have mutually interfered with each other. 



The normal alteration of orthoclase is accompanied by the development 

 of scaly or granular matter. The scales may often be definitely recognized 

 as a white micaceous mineral (sericite or muscovite) possessing high double 

 refraction. The development of this scaly and granular matter is very 



