350 ■ B. Hohson — An Irish AugitUe. 



cannot be made out with certainty. It is also abundant in the 

 ground-mass. Not unfrequeutly it is partially altered to yellowish 

 brown limonite. 



Biotite is present in onlj' small quantity. Its pleochroisra is, for 

 rays vibrating parallel to the cleavage cracks, golden yellowish 

 brown, for those at right angles to that direction faint yellowish 

 brown. It not merely surrounds the magnetite of the first generation 

 but appears to have crystallized after the augite crystals of the 

 ground-mass, as the mica is allotriomorphic to them ; in short they 

 project into it as the felspars do into augite in ophitic structure. 



The chloritic substance above mentioned by Rosenbusch, as 

 probably replacing an original glassy base, is present in considerable 

 quantity. It occupies more or less isolated angular spaces between, 

 or surrounds all the crystalline constituents. 



As the result of a somewhat hasty examination at the British 

 Museum (Natural History) of Mr. Allport's section, No. 1900, of 

 trap (from the Upper Trap band) of Rathjordan,' I found it to 

 agree with that from Ballytrasna- in most respects, though I had 

 not time to make certain whether phenocrysts of olivine were 

 originally present, as both Allport and HulP agree in stating. If 

 so, it would be a limburgite. I take the " cells " mentioned by Prof. 

 Hull, as occurring in the glassy base of the Ballj^trasna rock, and 

 the "tubes and cells" described and figured by him (pi. viii. fig. 9) 

 from the Rathjordan rock to be really augite crystals of the ground- 

 mass. That there is no improbability in limburgites (and augitites) 

 being present among the Limerick traps is shown by a remark of 

 Sir A. Geikie,* who states (apparently on the authority of Mr. W. 

 W. Watts) that the upper basalts of Nicker Hill ^ contain " only 

 38-66 per cent, of silica" "thus approaching the limburgites." 



As most of the traps from the Limerick district contain plagioclase 

 there can be little doubt that the Ballytrasna rock is a basaltoid ^ 

 augitite. It is probably more basic than the basalt of the Lion's 

 Haunch, Arthur's Seat, Edinburgh, and than the Manx melaphyre 

 with which I was led by Prof. Hull's description to compare it. 

 So far as I know, the Ballytrasna augitite (I use the term regardless 

 of geological age) is the first which has been described as occurring 

 in the British Isles. 



1 Described by Allport, loc. cit. p. 552. 



^ Ballytrasna must be a very small place, as it is not marked on the One-inch 

 Geol. Survey Map (sheet 15-1). ■ I understand that it is about two miles east of 

 Kalhjordan. 



3 Loc. cit. p. 157. 



* Anniversary Address to Geological Society, 1892; Proceedings p. 147 (p. 123 of 

 separate copies). 



= One-inch Geol. Survey Map, sheet 144 and Explanation, p. 31. 



8 H. Eosenbusch, Ueber die chemischeu Beziehungen der Eruptivgesteine. 

 Tschermak's Mineral, and Petrog. Mittheilungen, vol. xi. (^1889) p. 158. 



