246 



interstitial matter. As a general rule the ophitic rocks are holocrystalline ; 

 the augite playing the role of ground-mass. In this rock, notwithstanding 

 the presence of very large ophitic masses of augite with characteristic 

 inclusions and penetrations of lath-shaped felspars, there is a considerable 

 amount of interstitial matter. This consists of a brown isotropic glass 

 containing rough rods of magnetite, feathery microlites of felspar and 

 extremely fine acicular microlites, not capable of more precise definition. 

 This rock tills a volcanic neck according to Professor HULL. (I) It contains 

 the delessite-like mineral analysed by Mr. HARDMANN. and named by him 

 Hullite.' 2 ) 



Basic lavas and tuffs occur at two horizons in the Carboniferous 

 limestone of Limerick. They have been described by Mr. ALLPORT < 3 > and 

 Professor HULL.W They are basaltic in character and bear the closest 

 relation to the rocks of corresponding age in the midland valley of Scotland. 

 In the least altered rocks the ground-mass consists of minute lath-shaped 

 felspars, granules and crystals of augite and magnetite. Isotropic glass is 

 occasionally present. In this ground-mass plagioclase, augite and olivine 

 occur as porphyritic or micro-porphyritic constituents. The porphyritic 

 augites sometimes show exquisite zonal banding (rock from Ballytrasma) 

 and are honeycombed with inclusions. Mr. ALLPORT mentions the fact 

 that a rock from Ballybrod contains small patches of red and green 

 serpentine scattered through it. He says : " In one of the largest patches 

 there are several grains of olivine ; and an examination in polarized light 

 shows very clearly that it was originally a nest of olivine which has been 

 almost completely converted into serpentine." This is an extremely 

 important observation because it is the only one yet made of the occurrence 

 of anything like an olivine-nodule in any British basalt. Such nodules are 

 extremely common in the Continental Tertiary basalts, as for example, those 

 of the Rhine, and they have given rise to an interesting discussion. 

 LEOP v. BUCK, BISCHOF, SANDBERGER, PAUBRE'E and more recently A. BECKER 

 have maintained that the nodules are fragments or inclusions of rocks quite 

 distinct from the basalt ; ROTH, ROSENBUSCH and LASPEYRES, on the other 

 hand, hold that they must be regarded as segregations from the basalt- 

 magma. That the nodules in question are substantially identical in 

 structure and composition with the true peridotites (Iherzolites, saxonites 

 and dunites) is admitted on all hands. Thus, olivine is the most important 

 constituent and associated with this mineral we find enstatite, chrome- 

 diopside and chromite or picotite. Advocates of the inclusion hypothesis 

 rely on the fact that this association is not found in the normal basalt 

 and also on the frequent resemblance, in form and relation to the basalt 

 between the olivine-nodules and undoubtedly foreign inclusions such as 

 fragments of granite, sandstones, &c. Advocates of the segregation 



(1) Physical Geology and Geography of Ireland, p. 69. 



(2) Nature, 1878, Vol. XVIII, p. 507. Si0 2 39-43, A1 2 O3 10-35, Fe 2 3 20-72, FeO 3-69, 

 CaO 4-48, MgO 7'47, H.-0 12-61, Total 99'77. 



(3) Q. J. G. S., Vol. XXX., 1874, 552. 



(4) G. M., 1873, p. 153. 



