B. Sohson — An Irish Aiigitite. 349 



Boreslau, in Bohemia, with a section of which I compared it. 

 Professor Hull ^ describes the rock of Ballytrasna as containing; 

 "numerous large crystals and groups of banded felspar"; but I 

 failed to find a single felspar in four sections, nor did I observe any 

 in Allport's section, No. 1902,^ from the same locality, which agrees 

 with my sections. I submitted a section to Prof. H. Eosenbusch, 

 of Heidelberg, from whose letter to me I translate : " Plagioclase 

 I cannot find, it was most probably never present. In its early 

 state the rock contained only augite and iron ores in two genera- 

 tions, a little biotite around the iron ores of the first generation, and 

 probably a glassj' base, which, however, I nowhere with certainty 

 see preserved. On the contrary, it is changed to a chloritic sub- 

 stance, which is here and there speckled with brownish liraonite." 

 The structure of the rock is hypocrystalline porphyritic in Eosen- 

 busch's ^ sense. The phenocrysts * consist of augite and magnetite, 

 and perhaps biotite, if the inclusions of it seen in the augite 

 phenocrysts are not due to crystallization of included ground-mass. 

 The augite phenocrysts are idiomorphic and pinkish-brown in colour 

 by transmitted light. Their maximum diameter is occasionally 

 2-9 mm., but more commonly 1-65 mm. or less. Twinning is rare. 

 Zonal structure is common, so are magnetite and glass inclusions, 

 the latter often altered to the chloritic substance before mentioned. 

 The augite phenocrysts are themselves fi'equently partially or com- 

 pletely altered to a light-greenish serpentinous or chloritic alteration- 

 product, and the pseudomorjjhs thus formed are often surrounded by 

 interrupted patches of apparently secondary magnetite. Although 

 the zonal structure defines the crystal faces with distinctness, yet 

 the actual margins of the crystal sections are most frequently 

 ragged or jagged and irregular, with magnetite crystals, or more 

 rarely with augite crystals of the ground-mass, projecting into them. 

 This irregularity of contour is well shown in fig. 19 (pi. xxxiv.) of 

 Mr. S. Allport's paper.^ I am inclined to regard it as due to the 

 intratelluric augites continuing their growth during the period of 

 effusion, rather than to a growth after consolidation of the rock. 



The augite of the ground-mass occurs as very abundant idiomorphic 

 crystals, mostly elongated in the direction of the vertical axis c. 

 The average dimensions of the . crystals are "07 mm. x '01 mm. 

 They are faintly brownish in colour and are frequently twinned, the 

 twinning plane being the orthopinacoid. 



Magnetite is very abundant. It occurs as phenocrysts, often 

 included in the porphyritic augites, as before mentioned, and is 

 frequently idiomorphic, though in some cases the crystalline form 



1 Loc. cit. p. 157. 



^ In the British Museum (Natural History). 



3 Op. cit. pp. 339-340. 



* Terra proposed by J. P. Iddings, "On the Crystallization of Igneous Eocks," 

 Phil. Soc. Washington Bulletin, vol. xi. (18S9), p. 73, for "porphyritic con- 

 stituents" in Rosenbusch's sense. 



5 On the Microscopic Structure and Composition of British Carboniferous Dolerites, 

 Quart. Journ. Geol. Soc. vol. xxx. (1874), pp. 529-567. 



