Vol. 60.] ROCKS OF THE EORROWDALE VOLCANIC SERIES. 73 



rock, frequently associated with, a twinned muscovite. Pyrites 

 is common, in square sections giving a reddish transmitted light, 

 and is almost invariably surrounded by chlorite. Zircon occurs in 

 minute crystals, with the pyramid-planes developed. 



Very little can be made out of the fine-grained rock. Chlorite - 

 flakes are scattered throughout ; quartz, a few minute felspars 

 entirely altered to white mica, and aggregates of iron-ore also 

 occur in it. When this rock is taken up by the coarse rock of the 

 small exposure, it becomes dark-green and much more crys- 

 talline. A section across the two (3795) shows that the coarse 

 rock is a basic granophyre, containing turbid phenocrysts of 

 plagioclase in a micropegmatitic groundmass. Colourless fragments 

 of augite-crystals occur, surrounded by a mixture of chlorite and cal- 

 cite produced from their decomposition. Apatite is fairly abundant. 

 The dark-green rock might be described as a quartz-diabase. 

 The same turbid felspar occurs sparingly as phenocrysts in a 

 matrix of plagioclase, quartz, and chlorite. Sometimes the chlorite- 

 patches enclose the felspar in ophitic relation. Iron-ore is more 

 abundant than in the coarser rock, but apatite less so. 



Turning to the intermediate porphyritic types, we find 

 that the most basic (3752) contains quartz arid felspar-phenocrysts, 

 with garnet and pyrites. The last-named mineral is very abundant, 

 and occurs in small cubes, surrounded by a zone of quartz which may 

 consist of radial flakes or of little grains. Garnet occurs in rounded 

 sections, much corroded and surrounded by a ring of plagioclase. 

 Pseudomorphs of chlorite and epidote after original biotite are 

 common. The epidote is usually developed in grains along the 

 cleavage. The groundmass consists of felspar-laths, quartz, and 

 pyrites. 



The next type (3751 : PL XIII, fig. 1) contains phenocrysts of a 

 felspar, probably an acid labradorite, enclosed by flakes of biotite. 

 Xenoliths consisting of altered felspar, hornblende, and ilmenite 

 occur, closely resembling the hornblende-porphyrite to be described 

 later. Large blebs of quartz are seeu, much corroded, and conse- 

 quently surrounded by an aureole of a lighter colour than the rest 

 of the groundmass. A slide of this rock shows beautifully the 

 felspar-ring developed round a garnet. The garnet is much corroded, 

 and biotite occurs in the embayments; the whole garnet is sur- 

 rounded by plagioclase, with many prisms of apatite. Biotite is 

 often entirely decomposed to chlorite and iron-ore, lenticles of 

 calcite being developed occasionally along the cleavage. 



Slide 3776 differs from the last-described in the greater proportion 

 of felspar-phenocrysts, of quartz in the groundmass, and the smaller 

 quantity of iron-ore. The felspars show albite and pericline-twin- 

 ning, with occasional well-marked zonary banding. Extinction- 

 angles point to andesine or oligoclase-andesine. Paragonite-mica 

 and possibly free quartz result from them ; and in man}- cases the 

 crystals are completely transformed into these products, acquiring 

 at the same time a reddish tinge, which gives them the appearance 



