440 MR. B. nOBSOX ox THE IGNEOUS ROCKS 



formula K.^0 Alfi^ (8iO^)^,, and the soda as albito from fclie formula 

 Na^O Al^Og (SiOj)^, the following percentages are obtained : — 



Orthoclase iDcrcentage 12*6 



Albite percentage 32*0 



A section cut for the microscope from spec. 204 (centre of dyke) 

 consists of quartz, orthoclase, plagioclase, a microcline-like felspar, 

 white mica and dark mica. The bulk of the rock consists of a 

 granular aggregate of quartz, felspar, and mica, the average diameter 

 of the constituents being 0-2 millim. White mica is much more 

 abundant than dark mica. Larger, apparently porphyritic crystals 

 of plagioclase, microcline-like felspar, and white mica occur. The 

 porphyritic felspars are sometimes bounded by crystal faces, but 

 their boundaries are usually irregular. They are generally cloudy and 

 contain abundant flakes of white mica. The irregular boundaries of 

 the large crystals are probably due to dynamic metamorphism, as 

 undulose extinction is frequent. iS'o parallel structure is visible. 



In a section cut at right angles to foliation from specimen 205 a 

 (about 6 inches from the western edge of the dyke), we see that the 

 greater part of the rock consists of a mosaic of much smaller grains 

 than in 204 (average diameter 0'07 millim.). In this mosaic large 

 aggregates consisting partly of cloudy, often multiple-twinned, and 

 partly of water-clear, not visibly twinned felspar (the latter free from 

 minute inclusions, but containing large lath-shaped sections of white 

 mica) stand out like porphyritic constituents and often exhibit un- 

 dulose extinction. Solitary large lath-shaped sections of white mica 

 also occur, and are sometimes bent so that one end extinguishes com- 

 pletely while the remainder shows brilliant interference-colours. 

 Apatite occurs in the felspar of this section and Nos. 204 & 205. 



If the section be examined b}' ordinary light with a low power 

 (2-inch) it will be seen to be traversed by irregular subparallel 

 narrow dark lines looking almost like cracks. If these darker lines 

 be examined with a high power (^ inch) they will be seen to be due 

 to bands of minute black particles (magnetite ?) *, and it wiU be 

 observed that each band is accompanied by the development of white 

 mica tj which, though not confined to the dark bands, is much more 

 abundant there than elsewhere. 



Section 205, from the east side of the dyke, shows the contact 

 between the dyke and the slates. The micro-granite is very fine- 

 grained; the average diameter of the constituents is about '05 

 millim. "White or slightly greenish mica is very abundant in laths 

 •05 millim. long. Close to the junction with the schist brown 

 mica due to contact is developed. These smaller constituents show 

 a marked tendency to arrange themselves parallel to the line of 

 contact. 



* A similar arrangement of magnetite grains in an altered slate is figured and 

 described by Allport in Quart. Journ. Geol. See, vol. xxxii. (1876) p. 410, and 

 pi. xxiii. fig. (5. 



t Compare Eosenbuscb's description of " Quetscbzonen," 'Mikr. Physiogr. d. 

 massig. Gest.' (1887) pp. 43'.\ 440. 



