444 MR. B. HOBSON ON THU IGNEOUS ROCKS 



and in one case enclosed a pseudomorph after an olivine crystal. 

 The porphyritic plagioclase is, as a rule, remarkably fresh. 



These sections also contain large idiom or])hic porphyritic augite 

 crystals of violet-brown colour by transmitted light ; and pseudo- 

 morphs after idiomorphic porphyritic olivine. The last-named con- 

 sist chiefly of calcite traversed by irregular bands of serpentine 

 which has been formed along cracks in the original crystal. 



In the groiindmass magnetite or ilmenite occurs in two forms : — 

 1st. As small, apparently idiomorphic crystals (probably original 

 constituents), in abundance ; 2nd. As large irregular patches with 

 ragged margins. These patches are in places altered to opaque- 

 whitish leucoxene, and are perhaps secondary. The chief constituent 

 of the groundmass is plagioclase in small crystals of short lath- 

 shaped- section. Augite, often in idiomorphic crystals, occupies the 

 spaces between the felspar crystals, but is mostly altered to chlorite 

 and a dirty brownish granular decomposition-product. 



There are large amygdules in the rock, which consist chiefly of 

 radiating fibrous greenish material. 



The rock, both in the hand-specimen and under the microscope, 

 bears a considerable resemblance to the well-known porphyritic 

 olivine-basalt of the Lion's Haunch, Arthur's Seat, Edinburgh*, the 

 most obvious difference being the predominating lath-shape of the 

 felspars of the groundmass in the Manx rock. This resemblance is 

 a point of considerable interest, since Prof. Judd has shown t that 

 the volcanic rocks of Arthur's Seat are probably of Lower-Carboni- 

 ferous age; and Prof. Hull has described t a " porphj-ritic mela- 

 phyre " bearing " a close resemblance to the rock from the Lion's 

 Haunch" as occurring at Bailytrasna, and belonging to the ''Upper 

 Trap-band" a little below the basal shales of the Coal-measures. 

 As it is highly probable that the Manx rock is of Lower-Carboni- 

 ferous age, we appear to have evidence of the eruption of closely 

 allied rocks in the South of Scotland, the Isle of Man, and the West 

 of Ireland in the Carboniferous period. 



It ought to be mentioned that the specific gravity of the Manx 

 rock (2*77 to 2*81) does not agree very closely with that of the 

 Lion's Haunch rock, which is 2*92 to 2-93 ; but the relative abun- 

 dance of olivine may make a great difierence in specific gravity. 



(e) Tlie Pier ite-porphy rite. — A dark-green coarsely-crystalline rock 

 with large black porphyritic augite-crystals embedded in a compact 

 green groundmass. Specific gravity 3*02 to 3*04. 



A section cut for the microscope shows that the rock consists 

 chiefly of very large idiomorphic augite-crystals, brownish by trans- 

 mitted light. One of these, of rather over average size, measured 

 5 millim. from clinopinacoid to clinopinacoid X 3-8 millira. from 

 orthopinacoid to orthopiuacoid. Zone-structure is very well developed. 

 Successive parallel bands of brownish granular inclusions foDowing 



* Teall, ' Brit. Petrogr.' pi. xxiii. fig. L 

 t Quart. Journ. Geol. Soc, vol. xxsi. (1875) pp. 131-148. 

 \ 'On ... . Limerick Oarboniferous Trap-rocks,' Geol. Mae;. (1873) pp. 157 

 &158. 



