Vol. 60.] ROCKS OF TTTK HOKROWDALE VOLCANIC SERIF.-. x , 



sometimes very numerous ; but they are usually of small size, and 

 present a very fragmentary appearance in hand-specimens of the 

 rock. 



A section of this rock (3753 : PI. XIII, fig. -A) shows numerous 

 fragments of andesitic lava, with small felspars and much ragged 

 iron-ore. There may be one or two fragments of sedimentary 

 rock derived from the Skiddaw-Slate Series. The garnets are 

 obviously fragmentary, and are not found in the lapilli, but in the 

 andesitic groundmass. They sometimes have a spongy character. 

 Felspar-crystals are occasionally seen clinging to the margin of the 

 garnet. As the result of infiltration, greenish and almost isotropic 

 chlorite is found, together with needles of calcite, throughout the 

 slide. The iron-ore enclosed by the chlorite is extensively converted 

 into brown pleochroic sphene, Quartz occurs in oval fragments. 

 An analysis gave a silica-percentage of 54*3-1. 



Garnets probably occur in the higher members of the series. 

 This mineral was found in a coarse ash close to ' lava No. 6 ' on 

 Brown Knotts. Again, immediately to the south of Barrow-Beck 

 Waterfall, a bright-green ash with garnets occurs at the base of 

 the prominent scarp. The thick garnetiferous lava i Xo. 10 ' occurs 

 above it, and can be traced almost continuously from Barrow to 

 High Seat. 



This lava varies greatly in appearance — sometimes forming a 

 greyish-green rock with many felspars, and not unlike the intrusive 

 rock at Dock Tarn ; at other times it is of a very dark green, and 

 the felspars are less numerous. A section of the latter type shows 

 phenocrysts of plagioelase, with Carlsbad and albite-twinning and 

 good zonary banding (3755), in a very fine andesitic groundmass. 

 Flakes of biotite highly charged with magnetite occur, and these 

 undergo change into chlorite, calcite, and iron-ore. The iron-ore 

 (ilmenite) gives rise to sphene peripherally. Garnet occurs abun- 

 dantly (3782, 3829), and often shows the ring of felspar, with 

 detached fragments of garnet lying in the centre of the plagioclase- 

 crystal. The centre of the garnet is also frequently corroded, and 

 occupied by felspar. Iron-ore is always separated, in other cases, 

 the garnet simply undergoes a peripheral change to chlorite. Ward 

 mentioned the occurrence of augite. Apatite is often seen. The 

 rock is a typical andesite, and has a silica-percentage of 61*35.' 



Ward mentioned a higher lava ■ Xo. 12,' as containing garnets, 

 and he gave an analysis of it, the percentage of silica being 59-511. 2 



Garnet-bearing ashes and breccias are very abundant on the hill- 

 side behind the hotels at Rosthwaite. Highly-cleaved garnetiferous 

 breccia occurs in Frith Wood, and an alternating series of garnet- 

 lavas and breccias extends from Brand Fell southward to Dock 

 Tarn. On Brund Fell itself a fine-grained greenish ash with good 

 banding is seen ; this is the common greenish ash of the district. 



'■ Harker, : Chemical Notes on Lake-District Rocks ' in the ' Naturalist ' for 

 1809, p. 57. 

 - Quart, Journ. Geol. Soc. vol. xxxi < 187-3) p. 406. 



