Vol. 52.] ON THE KILDARE INLIER. 603 



It will thus be seen that the rocks of Grange and Allen Hills are 

 very closely allied. On both hills we meet with : — (1) rocks which 

 have large aggregated masses of augite-crystals ; (2) rocks with 

 what are probably pseudomorphs after olivine ; (3) rocks with large 

 labradorites ; (4) rocks with the groundmass largely formed of 

 small felspars with square or rectangular cross-sections ; (5) rocks 

 with bastite-pseudomorphs after a rhombic pyroxene ; (6) rocks with 

 altered felspars, whose margins are made up of fresh secondary 

 felspar ; (7) rocks with plentiful spots of chalybite ; (8) rocks with 

 large vesicles filled with a chloritic mineral or serpentine in the 

 centre, and round the margin with quartz, which is often partially 

 spherulitic and sometimes encloses very perfect little spherulites. 



There is thus plenty of evidence that the igneous rocks of the Hill 

 of Allen are very similar to those of Grange Hill ; and although no 

 sedimentary rocks are exposed now on the Hill of Allen, we consider 

 it quite certain that these igneous rocks are of Bala age, and that 

 they are all of the nature of lava-flows. 



The greater area covered by these rocks on the Hill of Allen, as 

 compared with Grange Hill, may best be accounted for by an 

 alteration in their dip due to a fault between the two hills, running 

 in a direction more or less parallel to the fault at the base of the 

 Chair of Kildare. The beds of the Hill of Allen differ, however, 

 from those of Grange Hill not only in the amount of their dip, but 

 also in its direction, for they appear to dip to the north-west. 



VIII. Summary of Conclusions as regards the 

 Igneous Rocks. 



Taking Grange and Allen Hills together, four groups of rocks may 

 be made out : — 



(1) Rocks with abundant felspar-needles in the groundmass, 



showing good fluxion-structure : as, for example, the ande- 

 sites of the northern foot of Grange Hill. 



(2) Rocks with the groundmass largely formed of fair-sized, 



often fresh, square- edged crystals, including, as a rule, also 

 rhombic pyroxene and patches of haematite : as, for ex- 

 ample, the greenish hypersthene-augite-andesite [67, 78] 

 high up on the Hill of Allen and sometimes of Grange Hill. 



(3) Rocks with a groundmass formed of a crystalline matrix of 



felspar and augite, the latter not in definite crystals, but 

 filling up the spaces between the felspars ; very large • 

 labradorites and some serpentine present: as, for example, 

 the porphyritic basalts of both Grange and Allen Hills 

 [5, 73]. 



(4) Rocks with a fairly plentiful groundmass consisting of 



felspar-needles and some augite and magnetite ; the 

 porphyritic constituents consist of much-altered felspars 

 and large fresh augites, often aggregated together in 

 groups : as, for example, the basalts found along the eastern 

 base of the Hill of Allen and some of those of Grange Hill. 

 Q. J. G. S. No. 208. 2 t 



