:;s4 B. N. PEACH AND J. HORNE ON THE 



tion from a dyke close by the former locality exhibits a similar ground mass, 

 but more coarsely crystalline, which is traversed by veins of secondary quartz 

 and laminae of chlorite, with magnetite grains more abundantly developed. 

 All the 'specimens have been much kaolinised. 



The rhyolites possess certain microscopic characters which readily distin- 

 guish them from the preceding group. They exhibit fluxion structure of a 

 more or less perfect type and possess a micro-crystalline base. In this base, 

 crystals and grains of quartz and felspar are arranged in parallel but wavy 

 lines, round which the felsitic matter curves in continuous streams. The 

 parallel and wavy bands of felsitic matter vary considerably in density. Some 

 of them are extremely fine-grained, parts of them remaining dark under 

 crossed nicols. The coarser bands do not remain dark with crossed nicols, 

 and with polarised light many of the particles exhibit the play of colours 

 characteristic of quartz. It is evident, therefore, that much of the base of the 

 coarser bands consists of minute grains of quartz. The felspar and quartz fre- 

 quently exhibit rounded edges in the midst of the streams of devitrified mat- 

 ter. Another feature of these ancient rhyolites is the absence of microlites 

 in the ground mass, which are so characteristic of the younger vitreous 

 rocks. 



A section (59 e) from the north-east corner of Papa Little exhibits most of 

 the foregoing characters. The wavy bands of micro-felsitic matter are ad- 

 mirably shown, along which are arranged lines of quartz and felspar, but not 

 continuously. Plagioclase is of less frequent occurrence than the orthoclase, 

 and many of the quartz and felspar crystals have distinct rounded edges. A 

 few crystals of iron pyrites occur in the section, which decompose into fine 

 grains of limonite. A green dichroic mineral, probably hornblende, occurs in 

 one of the denser bands. There are also minute grains of viridite, which 

 are evidently decomposition products. No microlites are observable in this 

 section. In one section (2 e), taken from a dyke close by the foregoing instance, 

 the bands of fine micro-felsitic matter enclose small oval-shaped masses of a 

 coarse-grained material otherwise similar in character to the surrounding base. 

 The quartz is frequently elongated in the direction of the flow of the molten 

 matter. Throughout the base there are abundant dark grains, probably 

 opacite. The section (3 e) from the same neighbourhood possesses a very fine- 

 grained and dense band of micro-felsitic matter, in which a prism of orthoclase 

 lies obliquely across the direction of the flow, round which the stream of 

 devitrified matter curves in continuous lines. With crossed nicols, this dense 

 band exhibits patches which remain dark when the stage is rotated. 



The section from near Skeld ,in Sandsting, close to the edge of the granite 

 mass, exhibits very distinct fluxion structure, but the micro-felsitic base con- 



