301 



very irregular. Gulfs and fiords (fig. 2, PI. XXXI.), as it were, penetrate into 

 the substance of the mineral and are now occupied by the ground-mass of 

 the rock. In studying thin sections it is often difficult to be certain whether 

 a mass which is surrounded on all sides by quartz is really an inclusion or 

 Avhether it is a section of an intruding arm cut so as not to show its 

 connection with the ground-mass of the rock. The rounding of the angles 

 of the quartz crystals in the trachytic rocks and the penetrations of the 

 matrix are generally regarded as evidence of a corrosive action of the magma 

 upon the earlier formed crystals. In rocks which contain two generations 

 of quartz that of the second generation may occur in the form of grains 

 (e.g., micro-granite) or intergrown with orthoclase in micro-pegmatite (e.g., 

 granophyres). Quartz is one of the most stable minerals and accordingly 

 we find that the quartz of granites and gneisses has contributed very largely 

 to the formation of sedimentary rocks. Under the influence of dynamic 

 metamorphism it breaks up into a mosaic. This is strikingly exhibited 

 where gneisses have been formed by the deformation of granite. The 

 granular character of the quartz is then easily recognizable with a lens. 

 Under the microscope the grains of quartz which occur between the larger 

 constituents are associated with grains of felspar of the same dimensions 

 and the two minerals then form a granulitic aggregate. 



Tridymite. This form of silica is present in certain trachytes and 

 liparites. It is often associated with opal and hyalite. It is best recog- 

 nized in the drusy cavities, where it occurs in the form of flat hexagonal 

 tables which do not as a rule measure more than 1 mm. across. The tables 

 often overlap like the tiles on the roof of a house. As a constituent of 

 the ground-mass tridymite occurs only in very minute plates. In this 

 condition it may be recognized by the imbricate or tiled arrangement Avhen 

 a tolerably high magnifying power is employed. At ordinary temperatures 

 tridymite behaves as a biaxial mineral and for some time was supposed to 

 be triclinic. MERIAX^ has shown that it becomes uniaxial on heating and 

 it is accordingly now referred to the hexagonal system. Its double 

 refraction is positive. Its specific gravity varies from 2'28 to 2*33 and is 

 approximately the same as that of fused quartz. 



Tridymite appears to be produced by the actions of vapours and 

 solutions during the final stages of the consolidation of the rock in which 

 it occurs. The researches of FRIEDEL and SARASIN prove that both 

 tridymite and quartz may be produced in the wet way ; the former at a 

 higher temperature than the latter. Tridymite was recognized by Professor 

 VON LASAULX W in the liparite of Tardree, Co. Antrim, and this remains 

 at present the only recorded instance of the occurrence of this interesting- 

 mineral in the British Isles. 



Micfi. The micas of the present group of rocks vary considerably in 

 character and composition. They may be broadly separated into two 

 groups ; the dark coloured or black micas and the light coloured or white 



(1) N.J., 1884, I., 193. 



(2) Jour. Roy. Geol. Soc. (Ireland), New Series, Vol. IV., 1876, p. 227; and also T.M.M., 

 NeueFolge, Vol. I., p. 410. 



