Vol. 60.] ROCKS OF THE BORKOWDALE VOLCANIC SERIES. 75 



in the groundinass. Epidote is abundant, partly arising from 

 infiltration and partly from decomposition of original minerals. 

 This rock bears a strong resemblance to those found at Burtness 

 Combe, Buttermere. 



Perhaps enough has been said to give some idea of the petro- 

 logical character of these rocks. Their chemical relationships are 

 not less interesting. I have made partial analyses of four pro- 

 minent types, and these are sufficient to show how closely allied 

 the rocks are one to the other. These analyses were carried out 

 in the laboratory attached to the Mineralogical Museum, where, by 

 the kindness of Prof. Lewis and Dr. Hutchinson, every facility was 

 afforded to me for the work. 



A. B. C. D. 



Per cent. Per cent. Per cent. Per cent. 



SiO 57-91 60-02 61-63 6440 



TiO" ) 



Al.,0, \ 27-56 25-36 22-34 22-30 



Fe:,(X j 



MnO' 032 0-29 022 0'14 



CaO 6-19 397 4-40 227 



MgO 1-20 0-91 0-98 0-60 



K.,0 1-82 2-55 4-04 579 



Na,0 1-64 2(52 251 2-51 



Specific gravity 2*856 at 13-8° C. — 2-748 at 13-4°. 2*722 at 16-1°. 



A= Fine-grained black rock of Blea-Crag summit. Slide 3750. 



B=Quartz-garnet-mica-porphyrite in stream above Blea Crag. Slide 3751. 



C = Quartz-garnet-mica-porphyrite in stream above Blea Crag. Slide 3776. 



D = Granophyre from Langstrath Beck, south of the Stake Pass. Slide 3787. 



A specimen of A taken close to the acid segregation gave 60*51 

 per cent, of silica ; and a dark-green porphyritic rock with pinkish- 

 white felspars, intermediate between B and C, yielded 60-49 per 

 cent, of silica. 



A very important feature brought out by these analyses is the low 

 percentage of magnesia in all of them. The higher percentage of 

 lime in C (as compared with B) is easily explained, when sections of 

 the two rocks are examined. The former contains a large amouut 

 of calcite which has, to a great extent, been introduced by infiltration. 

 In rocks that have undergone such great alteration, it is scarcely 

 to be expected that chemical analysis will bring out that gradation 

 which is so well shown in the field. With regard to the alkalies, 

 the preponderance of potash over soda is hardly to be looked for, 

 seeing that orthoclase is not found in large crystals in the inter- 

 mediate rocks. It is doubtless well represented in the groundinass. 

 The low percentage of alkalies in the dark fine-grained rock A is 

 remarkable, and may be explained as the result of differentiation. 

 A section shows concentration of iron-ore. The high percentage of 

 silica may be due to infiltration. 



All these rocks are plentifully veined with quartz and chlorite, 

 or quartz and epidote. A yellow fine-grained rock is often met 



