OF THE SOUTH OF THE ISLE OF MAN. 44.'} 



by chlorite and sometimes caleite. The originally vitreous material 

 forminti: their walls contains abundant grains of leucoxene. 



B. Fragments almost as full of vesicles as A, but in which the 

 glass is rendered brown by very fine dust-like inclusions. Larger 

 acicular dark-coloured rods, white by reflected light, probably 

 leucoxene, are abundant, and occasionally a lath-shaped plagioclase 

 crystal may be seen. 



C. Scoriaceous fragments in which the vesicles and the solid part 

 occupy about equal space. In these fragments the vesicles vary 

 greatly in size from less than '05 to 2*87 millim. in diameter, 

 whereas in A and B they are fairly uniform in size, and al)out 

 •1 millim. in longer diameter*. The cavities of the scoriaceous 

 fragments are occupied in most cases by chlorite, in some cases by 

 chlorite and caleite. The solid part contains very small lath- 

 shaped plagioclase and abundant iron-ore. None of it is isotropic ; 

 probably this is due to secondary alteration. 



The fragment of type C appears to be part of a small ejected 

 block rather than a normal integral part of the tuff. 



The fragments are cemented by a calcareous cement. Specimens 

 33 and 200 differ considerably from 500 in maeroscopical appearance, 

 being hard breccias in which greenish fragments of augite-porphyrite, 

 sometimes as much as an inch across, are embedded in a grey calca- 

 reous matrix. Microscopically they are seen to consist chiefly of 

 scoriaceous fragments of the type C just described. Idiomorphic 

 crystals of iron-pyrites are present in the amygdules of 200. 



Sections IGO and 502 a (both from ejected blocks in the agglo- 

 merate) agree with type C of section 500 in structure, but the 

 amygdaloidal cavities are smaller and much more crowded, and the 

 felspar laths more abundant. In section 160 pseudomorphs after 

 porphyritic olivine are present, and the rock is seen to consist of a 

 breccia in which fragments of a previously-solidified very vesicular 

 lava are embedded in similar lava with evident fluxion-structure, 

 the slender felspars being arranged parallel to the contours of the 

 included fragments f. 



(d) The Mtlaphyre Bijlce. — Large plagioclase- and augite-crystals 

 and pseudomorphs after olivine are embedded in a compact grey 

 groundmass. The specific gravity of three specimens was respec- 

 tively 2*77, 2'78, 2-81 ; average 2-79 (the specimens were not free 

 from amygdules). 



Sections 501 and 501 b i, cut for the microscope, show large 

 idiomorphic porphyritic crystals of plagioclase. In some of the 

 plagioclase crystals the twinning lamelloB terminate abruptly at 

 cracks, or are only faintly indicated at the other side of cracks in 

 the crystal. In these crystals undulose extinction was observed. 

 We may therefore consider the twinning in these cases to be due to 

 pressure §. The crystals are in some cases eroded by the groundmass, 



* In A much larger vesicles do occur. 



t Compare Fouque and Levy, ' Mineral, niicrogr.' (1879) pi. xxxii. 

 I Loos^e block on shore, see iDife. 

 ■ § See Judd, Quart. Jouru. Geol. Soc. vol. xli. (1S83) p. 3GG; Furstner, Zditschr 

 fiir Krystallogr. vol. is. (1684) p. '6'i'6. 



2^2 



