340 



felsitic matrix. Analysis by J. HUGHES. Q.J.G.S., Vol. XXXI, p. 400. The analysis is made 

 up to 100 by -089 of phosphoric acid and '017 of sulphuric acid. 



III. Pale green felsite near Pitt's Head. Analysis by HAUGHTON. Trans. Roy. Irish 

 Acad., Vol. XXIII., p. 615. 



IV. Matrix of nodular felsite from the Lledr Valley, near Conway Falls Inn. Q.J.G.S., 

 Vol. XXXIX, p. 485. Analysis by HATCH. 



V. White spheiulitic rock. Digoed near Penmachno. Q.J.G.S., Vol. XLII, p. 187. 

 Analysis by COLE. 



VI. Black layer in large spherulite, Digoed. Substance allied to pinite. Analysis by COLE. 

 VII. Orthofelsite. Arenig. Analysed by Mr. PLAYER, in the author's laboratory. 



Midland Counties. Near Wellington, in Shropshire, occur the re- 

 markable devitrified rhyolites and rhyolitic tuffs described by Mr. ALLPORT/ 1 

 These rocks are probably of pre- Cambrian age ; at any rate they are 

 overlain unconformably by stratified rocks as old as the Holybush sand- 

 stone. They are of great importance in the history of petrography 

 because they enabled Mr. ALLPORT, in 1877, to demonstrate the existence 

 in very early times of perlitic and spherulitic hyaline rhyolites, identical 

 with those of Hungary. The most striking variety described by Mr. ALLPORT 

 from the " Lea Rock " quarry " consists of numerous bright red spherulites 

 set in a grey or yellowish green matrix." The spherulites sometimes occur 

 singly ; more frequently they are aggregated together in patches or bands. 

 Where they have been freely developed they usually show, in thin section 

 " a circular central disk of bright red surrounded by a colourless (or pale 

 red) ring ; the latter varies greatl} 7 in width and is perfectly continuous 

 with the red portion, of which it is the unstained border." They are 

 radially fibrous and give a black cross. In the character of their double- 

 refraction these spherulites differ from the small colourless spherulites 

 found in many felsites. They are positive; in other words they behave 

 as if they were composed of uniaxial and positive fibres. In the aggregate 

 of spherulites the development of a regular and radially fibrous arrange- 

 ment is considerably interfered with, and very often no definite structure 

 is recognizable. The red bands are frequently seen to widen out at their 

 terminations. Where the radial structure is imperfect the double-refraction 

 is scarcely, if at all, recognizable. A few porphyritic crystals of felspar 

 occur in the rock. These have exercised no influence on the grouping 

 of the spherulites. The substance which has formed the spherulites is 

 the micro-felsite of ROSENBUSCH. In their internal structure and relation 

 to the surrounding matrix the spherulitic bodies are identical with those 

 of the rhyolites (lithoidit) of Szanto and Tolcsva in Hungary/ 2 ^ 



The matrix in which the porphyritic crystals and spherulitic bodies 

 lie is devoid of the red colouring matter (ferrite). It often shows perfect 

 perlitic structure. The perlitic cracks are frequently occupied by a green 

 substance (viridite or chlorite) and the same substance is often scattered 



(1) Q.J.G.S., Vol. XXXIII., 1877, p. 449. 



(2) VOGELSANG, Die Krystalliten, p. 147. 



