CONDITION OF THE QUARTZ IN RHYOLITES. 271 



character, or the meshes may disappear altogether, leaving simple 

 aggregations of sphserolites, or the meshes may expand into confused 

 aggregations of bunches of parallel fibres ; or a felt-like structure 

 may be formed of short confused fibres, or the sphserolites may be mixed 

 with aggregations of cumulites. Other rhyolites consist of aggre- 

 gations of colourless polarising particles and colourless glass, but these 

 are rare. Alternating bands of microfelsite and light-coloured glass 

 are also rare ; but it is commoner to find a half-glassy mass made up 

 of little thin microliths almost passing into obsidian, but containing 

 crystals of quartz, sanidine, and biotite. The homogeneous glass of 

 other rhyolites is sometimes contorted and undulated with dark- 

 brown grains ; or spherolites and axiolites may replace the glass, and 

 hajft fluxion structure marked by similar bands of grains ; or the 

 lignt-coloured homogeneous glass may be traversed by perlitic cracks, 

 and have a fluxion-structure marked by narrow zones of microfelsite 

 on both sides. 



Characteristics of Rhyolites. Rhyolites are best characterised 

 by fluid structure and fibrous aggregates. The wavy fluidal struc- 

 ture is due to several causes. First, coloured bands are formed by 

 parallel layers of needles and grains of ferrite and opacite. Secondly, 

 the different layers of the rock may vary in texture, as when crystal- 

 line granular layers alternate with spherolite layers ; or when micro- 

 felsitic layers alternate with less crystalline layers ; or perfectly 

 granular layers with imperfectly granular layers. The corrugations of 

 fluidal structure are also sometimes marked by the alternations of 

 layers of colourless glass with brownish-yellow globulitic glass. 



The fibrous concretions of rhyolites comprise four types : first, 

 those in which the fibres radiate from a centre as in spherolites ; 

 second, those in which the fibres are arranged longitudinally about 

 an axis forming axiolites; third, those in which the fibres are 

 parallel to each other forming bunches or bundles ; and fourthly, those 

 in which the fibres are confusedly mixed. These fibrous aggregates 

 are wanting in trachytes. 



Khyolites are. even better characterised by their mineral composi- 

 tion. The crystalline quartz component is a consequence of the high 

 percentage of silica in the rock ; being often in excess of what could 

 be used up in forming felspar, some has crystallised separately. 



Quartz in Rhyolites. The quartz of rhyolites may be in frag- 

 mentary grains, or in double six-sided pyramids, divided by a six-- 

 sided prism, the latter condition being developed in proportion to the 

 crystalline texture of the rock. The glass inclusions sometimes have 

 this crystalline form, and then contain dark bubbles ; but the moving 

 bubble and the fixed bubble are never found in the same quartz. 

 The quartz is poor in mineral inclusions, but contains ovoid gas 

 cavities, and ovoid inclusions of colourless glass, in which are opaque 

 needles. Microliths occur, and fluid inclusions have been described 

 in rhyolites from Lipari, Samothrace, the Auvergne, and Kis Sebes, 

 but they are evidently rare ; for among all the rhyolites of North 

 America, only two were found with fluid inclusions in the quartz, 



