342 



I. & IL Duplicate analyses by Mr. PHILLIPS of the devitrified perlitic pitchstone from the 



" Lea Rock " quarry. Q.J.G.S., Vol. XXXIII., p. 457. 

 II. Perlite from Hlinyik, near Schemnitz. Analysed by VON SOMMARUGA. 

 III. Perlite from same locality. Analysed by RAMMELSBERG.(I) 



On comparing the above analyses it will be seen that the devitrified 

 rocks contain less water. There is also a marked difference in specific 

 gravity. In other respects there is the closest possible resemblance 

 between the ancient and modern rocks. The significance of the difference 

 in specific gravity has already been discussed (see ante page 13). 



The Lake District. Rocks of the group now under consideration occur 

 in this district in two forms (1) as dykes and intrusive masses and (2) as 

 lava-flows. The former have been described by Mr. WARD in his memoir 

 on the northern part of the Lake District. The Armboth dyke is com- 

 posed of a biotite-quartz-felsite. Crystals of pink orthoclase give the 

 rock a marked porphyritic character. Plagioclase is also present. The 

 ground-mass shows micro-pegmatitic or pseudo-spherulitic structures so that 

 the rock is a granophyre in Rosenbusch's sense. Here and there it 

 contains minute but well-formed dodecahedra of garnet. The occurrence 

 of garnet and pinite (pseudomorph after cordierite) in the quartz-porphyries 

 is interesting because these minerals occur also in the liparites.< 2) The 

 latter mineral is not known in the Armboth dyke but it has been found 

 in some of the Cornish " elvans." Mr. WARD records the occurrence of 

 several other intrusive quartz-felsites in the Lake District. They are 

 probably connected with the masses of granite. 



The rock from the summit of Carrock Fell is a beautiful example of 

 an augite-bearing granophyre. This is the spherulitic felsite of WARD. 

 It stands in close relation to the quartz-gabbro or augite-diorite. 



A mass of quartz-felsite is extensively quarried near Threlkeld. It 

 contains porphyritic quartz and felspar, with some dark mica, in a crypto- 

 or micro-crystalline ground-mass. The lavas are associated with the 

 Coniston limestone. They correspond therefore with the Ordovician felsites 

 of North Wales. Mr. RUTLEY has described some of them in his memoir < 3) 

 on " The Felsitic Lavas of England and Wales." 



Perlitic structures have been observed in felsites from the Red Crag, 

 one and a half miles N.E. of Stockdale in Westmoreland, the northern 

 end of Long Sleddale Valley, the neighbourhood of Grizedale Tarn and 

 the Copper Mine Valley, N. of Coniston. In a rock from the Long 

 Sleddale Valley devitrification has resulted in the formation of small 

 positive spherulites giving a well-defined black cross (see Plate XXXVIIL). 

 This type of devitrification is associated with the more common micro- 



(1) Analyses II. and III. are given in Mr. ALLPORT'S paper. They are quoted from ROTH'S 

 Beitrage zur Petrographie, 1869, and Gesteins Analysen, 1861. The specific gravities of the 

 rocks analysed are here added. 



(2) SZABO. Der Granat und der Cordierit in den Trachyten Ungarns. N. J. Beilage 

 Band I. 302. 



(3) See also Q.J.G.S. Vol. XL. (1884), p. 344. 



