536 MR. F. RUTLEY ON SOME OF THE 



A dark bluish-grey, vesicular rock, the vesicles being mostly filled 

 with pale greyish-white or dark green substances. The weathered 

 surfaces are rusty-brown and the amygdules at the surface have 

 usually decomposed, leaving empty pores which impart a somewhat 

 Bcoriaceous aspect to the rock. Under the microscope, in thin section, 

 it is seen to consist of once glassy matter, now more or less devitrified 

 by the development of microliths and globulites, and crowded with 

 small granules and crystals of magnetite, together with little needles 

 and skeleton crystals, often twinned, and these from their extinction- 

 angles appear in many instances to be labradorite. There are also 

 many small lath-shaped crystals of triclinic felspar often considerably 

 corroded (PI. XIX. fig. 2). One or two of the skeleton crystals are 

 shown in fig. 7, magnified 75 diameters. Fig. 6 in PI. XIX. re- 

 presents what appears to be a diminutive pseudomorph of magnetite 

 after olivine, magnified 140 diameters. Taking the boundaries of 

 the crystal-section as the brachydome (021) and the brachypinacoid 

 (010), the angle for the faces 021 : 021 approximates to 80°. 

 Minute pale greenish grains are also present which may consist of a 

 dusty admixtxire of chlorite with some other mineral. 



The devitrified glass in the thinnest parts of the section appears 

 nearly colourless ; where thicker it is of a pale brown tint. 



The vesicles in this rock are exceedingly numerous and very 

 irregular in form (PI. XIX. fig. 1). They are filled with quartz, a 

 clilorite-like mineral (apparently delessite in part), calcite, and some- 

 times chalcedony and a substance resembling felsitic matter. The 

 larger grains in these microcrystalline aggregates usually show a 

 positive uniaxial interference-figure, but occasionally the emergence 

 of a negative bisectrix may be noted in convergent light, indicating 

 that a biaxial mineral (probably felspar) is also present. 



Quartz, chalcedony, or felsitic matter frequently borders the 

 vesicles, the interior being filled with calcite only, with quartz only 

 or with chlorite (delessite ?) only, but often with an admixture of 

 chlorite or delessite and (juartz. The skeleton crystals are mostly 

 of the H-shaped or swallow-tailed types. They very commonly 

 show straight extinction, but in many cases the direction of maxi- 

 mum extinction makes angles with the axis of elongation varying 

 from 12° to over 15°. 



The rock appears to be closely allied to certain Bohemian 

 melaphyres described by Bof icky *. It doubtless represents the 

 once vitreous, superficial portion of an old lava-flow of basalt or 

 andesite, and is probably one of the most ancient examples of such 

 a rock with which we are yet acquainted. 



No. XIV. S.W. side of Caer Caradoc, at the top of the hill. 

 (Melaphyre Tuff.) — The fragments composing this tuff consist of a 

 rock similar to that just described. They are mostly small, ranging 

 from three or four millimetres in diameter to very minute dimen- 

 sions. The sections of the fragments frequently show concavities on 



* ' Petrographische Studien an den Mclapbyi'gesteinen Bdhmens,' Archiv fur 

 Natiirw. Landesdurchforsch. v. Bohmen, Bd. iii. Geol. Abth., Prag (1876). 



