160 Prof. Hull — On the Structure of Trap-rocks. 



described may be regarded as the more ancient equivalents of the 

 Tertiary basaltic sheets, it may be interesting to compare their re- 

 spective structures. 



I have examined a considerable number of sliced specimens taken 

 from various parts of the Antrim trap-area, and it is remarkable that 

 they all present a nearljj^ uniform structure, notwithstanding different 

 degrees of crystallization. In this case the augite, instead of being 

 imbedded in a crystalline form in a colourless glassy base, is itself 

 the base, inclosing prisms of triclinic felspar, grains of magnetite, 

 or titano-ferrite, and sometimes olivine. Chlorite is also present, but 

 not so frequently as in the Limerick melaphyres, and the whole 

 rock is in a less altered state. Thus, if we compare the composition 

 of the Limerick and Antrim traps, we would find that the relation- 

 ship is somewhat as shown below : — 



Eelative proportions of minerals in the Melaphyres of Limerick and the 

 Basalts and Dolerites of Antrim, Down, etc. 



Limerick. Carboniferous. Antkim. Tertiary. 



Base ( -^T^igite crystals. 



■ ) Triclinic felspar, Magnetite. 

 Colourless glass, (^Yxviue (Pseudomorphs). 



containing y Calcite& chlorite (accessories). 



Base. ( Triclinic felspar. 



. . 1 Titano-rerrite. 



Augue, < Olivine (sometimes), 



containing ( chlorite (sometimes). 



It will be thus seen that the relations of the minerals are quite 

 different in each case, and it seems probable that this is due to the 

 greater abundance of augite in the Antrim traps ;— so abundant as to 

 have itself become the base. In the case of the Limerick melaphyres 

 the mineral occurred in such small quantities that it crystallized out 

 in individual crystals, bearing a small proportion to the whole mass 

 of the rock. This points to essential differences in the two magmas, 

 involving some interesting chemical inquiries. 



The distinction here stated between the structure of the Carbon- 

 iferous and Tertiary augitic rocks (melaphyres and basalts) is not 

 admitted by Mr. AUport, with reference to similar rocks of England 

 and Scotland. Mr. AH port's opportunities of investigation, however, 

 have been much larger than my own, and the result he arrives at 

 may be thus summed up in his own words : — 



"I have abundant evidence that melaphyres of undoubted Carbon- 

 iferous age, and basalts of Tertiary age, have not only the same 

 mineral constitution, but also that both present the same structural 

 varieties." ^ My observations tend rather to confirm the opinion of 

 Mr. Forbes, as expressed in his paper " Researches in British Miner- 

 alogy." ^ And the glass base of the Limerick melapyhres is in keeping 

 with the general character of the basaltic rocks as described by Dr. 

 Zirkel.3 



EXPLANATION OF PLATE VIII. 

 Pig. 1.— Twin crystals of triclinic felspar, inclosing spangled greenish grains, prob- 

 ably of olivine. From Upper Trap-band, Ballytrasna. 

 Fig, 2. — Augite crystal, inclosing magnetite grains. From Upper Trap-band, 

 Ballytrasna. 



1 Geological Magazine, Vol. VIII., p. 449. 



2 Phil. Mag., vol. xxxiv., p. 336. ^ Sup. cit., p. 88. 



