13 



inclusions in ragged or rounded grains are irregularly dis- 

 tributed, at times forming a narrow rirn or frame surrounding 

 the inclusions of plagioclase or mica. A little apatite is also 

 seen, in needles and short stout prisms. 



All of these are primary inclusions, of earlier date than 

 the felspar, but other apparent inclusions of quartz and 

 sericitic muscovit-e are strung out in irregular lines and 

 evidently fill cracks or solution cavities. A fairly constant 

 characteristic of these felspar phenocrysts is the presence of a 

 rim of variable width composed of quartz granules graphic- 

 ally intergrown with the felspar and optically continuous 

 with each other over small areas, or at other times optically 

 independent. The inner edge of the rim is as a rule 

 straight. There are outorowths of the felspar beyond the 

 rim, the phenocrysts having as a result irregular boundaries 

 against the other minerals of the rock. These outgrowths 

 are usually notably free from perthitic intergrowths of albite. 

 The whole appearance suggests that the felspar oris^inally 

 separated out from a fluid magma in idiomorphic crystals, 

 and afterwards, under changed conditions, resumed growth, 

 the additions consolidating during the crystallization of the 

 ground-mass (see pi. i., fig. 1). It is this characteristic of the 

 phenocrysts that produces the deceptive effect, megascopically, 

 of rounding and resorption. 



The microcline of the second generation is not in great 

 amount, and is free from inclusions, and for the most part also 

 from perthitic intergrowth, though traces of this are occasion- 

 ally seen. 



Quartz. — The quartz of the earlier generation is sub- 

 idiomorphic at times, in crystals 'up to 1 cm. in length, and 

 preserving under the microscope their hexagonal appearance, 

 but with very ragged boundaries. Minute liquid and glass 

 inclusions are extremely abundant everywhere throughout the 

 crystals, occasionally aggregating into bands or strings and 

 often containing fixed or mobile gas bubbles. In addition 

 extremely slender dark rods, up to about 01 mm. long, are 

 very thickly distributed in places ; these may be rutile. The 

 quartz crystals show undulose extinction and are invariably 

 traversed by irregular cracks which break up the area covered 

 by any one section of quartz into a number of smaller areas, 

 each in different orientation from its neighbours, as if frac- 

 turing had been in some way accompanied by rotation of the 

 fragments. And yet strings of glass inclusions are often 

 continuous over two adjacent areas. Tlie cracks when cut 

 diagonally polarize in bright colours. 



A curious feature in connection with this irregular 

 cracking is that in thin section the quartz when viewed lb 



