Professor Geikie on the " Fitchstone " of Eskdale. 247 



frequently be observed in interspaces between groups of 

 felspar or augite crystals, vary in size from extremely minute, 

 microscopic portions up to these prominent kernels already 

 referred to as so conspicuous to the naked eye. Sometimes 

 streaks may be noticed proceeding from the clots, and 

 following a sinuous course among the crystals. In other 

 cases the clots themselves are made up of confluent streams 

 of dark-brown glass curving round and enclosing groups 

 of crystals or microliths. Curdled patches and streaks of 

 similar darker glass appear all through the rock, and by their 

 current-like flow between the felspars and augites show' with 

 great beauty and distinctness the former extremely fluid con- 

 dition of the mass. 



• The frequently observed bleaching of a volcanic glass round 

 its enclosed crystals may be here observed in many examples ; 

 likewise the contrary effect where the glass attains its blackest 

 opacity along the margins of crystals, especially of the felspars. 

 The bleaching may be observed more particularly round the 

 augite and magnetite, as if the colouring iron oxide had been 

 drawn out of the still unsolidified glass by the attraction of 

 these ferruginous minerals. 



It is evident from the beautiful fluxion structure of this 

 glass round its enclosed crystals of felspar, augite, and mag- 

 netite, that these minerals had already assumed crystalKne 

 forms while the rock still possessed great internal liquidity. 

 Large enclosures of the glass may be noticed in many of the 

 clear felspars. The devitrification of the rock occurs in its 

 incipient stages even in the most glassy parts, and can be 

 traced until the characteristic brown glass disappears among 

 the crowded crystallites. 



Crystallites. — Of these rudimentary forms of crystallisation 

 the sections of the rock examined by me present, at least, three 

 kinds — 1st, Globulites. What appears under a low power to be 

 a homogeneous, structureless, or sometimes slightly dusty glass 

 can in some places be resolved into an aggregate of globules or 

 granules so minute as just to be visible with a magnifying 

 power of 400 diameters. There appears to be a tendency 

 throughout the glass to assume this globulitic character. It 

 is only here and there, however, that the globulites become 



