828 MR G. W. TYRRELL ON 



orthoclase in quantity. The rock may then be regarded as a latite lava ; but in 

 the absence of extended sections it is possible that these fragments only represent 

 accidental sporadic concentrations of oligoclase in a trachyte. 



Rock-fragments other than trachyte are rare. Amongst those found are ortho- 

 phyres containing numerous small, uniformly sprinkled, altered grains of pyroxene ; 

 a cryptocrystalline quartz-trachyte ; a very few magnetite-blackened basaltic frag- 

 ments ; and scarce fragments of baked shale. No fragments of plutonic rocks were 

 found ; neither were true andesites. Andesine felspar is prominent as phenocrysts 

 in a few fragments, but in all these cases the groundmass is composed of orthoclase, 

 and mafic (ferromagnesian) minerals are sparse or absent. 



The paste in which the crystals and rock-fragments are embedded consists of tiny 

 chips of the same rocks mixed with comminuted felspars, turbid glass, sparse 

 magnetite, and some minute indeterminate substances. The glass fragments are 

 occasionally curved, but ash or " bogen-structure " is not well developed in most of 

 the slides, owing, perhaps, to the comparative rarity of vesicular material. The 

 groundmass is sporadically scapolitised. 



The proportions of broken minerals, rock-fragments, and paste vary considerably. 

 In the majority of the rocks the trachyte fragments make up the bulk of the material. 

 In others, however, the crystals and fine paste are dominant. In the latter types 

 spherical bodies consisting of cryptocrystalline silica are often found, and have been 

 referred to radiolaria. Occasionally these are pseudomorphed in scapolite. 



These tuffs are built almost wholly of igneous material, mostly of trachytic 

 affinities, and are therefore true trachyte tuffs. In general they are remarkably 

 fresh, notwithstanding the peculiar alteration to scapolite many of the felspars have 

 undergone. The scapolite occurs in irregular areas within the felspar crystals, both 

 plagioclase and orthoclase, also as apparently independent areas always anhedral in 

 form, and occasionally as an irregular replacement of the groundmass. Prismatic 

 bounding planes are occasionally seen, which are parallel to a good cleavage. The 

 mineral thus forms rude prisms, always with irregular terminations. Frequently, 

 however, it is in more or less rounded grains aggregated together and occupying the 

 space formerly held by a felspar crystal. The extinction is straight with the direction 

 of extension of the prisms where these are present, and also with a prominent 

 cleavage. This is doubtless the perfect cleavage parallel to 100 ; it is crossed occa- 

 sionally by another at right angles, parallel to 010, and both these are intersected 

 at 45° by a cleavage parallel to 110. The mineral polarises in bright colours similar 

 to those of thick quartz ; but frequently, in the same area, there are pieces which 

 have a much lower double refraction, which is, at all events, higher than that of the 

 felspar from which they have arisen. The prisms are optically negative in the 

 direction of extension. 



All these characters agree with those of scapolite. On the other hand, the 

 mineral is occasionally distinctly biaxial, and has simple or multiple twinning. The 



