26 



PKOP. T. G. bon:n'et on the 



mechanical causes, in the other to the action of intrusive igneoua 

 masses ; so that I may venture to speak with some confidence as to 

 the usual characters of metamorphic and hypometamorphic rocks. 



Microscopic examination of the gneiss* shows it to consist of quartz, 

 in rather small grains, felspar, occurring occasionally in larger, hut 

 very irregularly bounded grains, mica, brown, pale-olive green, and 

 colourless, and garnets, with some rods, flakes, and imperfect crys- 

 tals of black iron-oxide. The quartz is clear and calk for no remark. 

 The felspar does not exhibit any twinning structure, is in moderately 

 good preservation, and commonly includes flakes of mica, small 

 garnets, and sometimes granules of quartz. The larger flakes of 

 brown mica occasionally include small crystallites, sometimes, I think, 

 quartz, sometimes perhaps apatite. The olive-coloured mica is clearly 

 only the result of alteration of the brown. The colourless mica, as usual, 

 gives rich tints with crossed nicols. The garnets, generally about 

 •003" diameter, are practically colourless, but often so full of dusky 

 granular inclusions as to be darkened when looked at with a low 

 power. The sections prove them to have a well-developed crystal- 

 line form. There is no very definite mineral-banding, but the mica, 

 especially the white, tends to be arranged in irregular wavy lines, 

 imparting to the mass a moderate degree of foHation. Study of 

 this and of the rock generally, leads me to the conclusion that it has 

 undergone a certain amount of mechanical disturbance at a rather 

 remote epoch. Lithologically, the rock resembles one of the gneisses, 

 which, in the Alps, occur rather low down in the succession which 

 I, in common with many other geologists, believe to exist, though 

 still by no means at the bottom of the series. 



MatHcc of Conglomerate from Ohermittiueida, showing well-developed 

 mica jlahes associated ivitJi quartz (tJie white ground of the 

 figure), x 140. 



The matrix of the conglomerate is almost wholly composed of 

 three minerals, quartz, felspar, and mica, of which the first and 



* dm. Prof. Hughes's section, p. 22. 



