Vol. 52.] ON THE KILDARE INLIER. 599 



are several rocks among the more basic flows which approach it in 

 character. 



Not far south of this exposure of coarse rock — south and south- 

 west of the actual summit of the hill — occurs a patch of an andesite- 

 rock having a compact blue or black groundmass in which small 

 felspars can be detected on examination of a hand- specimen. Sections 

 of this rock resemble those of the lower andesitic flows. A ground- 

 mass of small lath-shaped felspars, very small augite-grains and 

 magnetite, is to be seen. The small felspars are allied to oligoclase, 

 and the groundmass shows here and there a perfectly spherulitic 

 structure. Fair-sized porphyritic felspars occur, much altered, but 

 probably labradorite. Epidote occurs occasionally, and a few 

 amygdules are to be seen with an outer rim of epidote and quartz, 

 the latter often showiug good spherulitic structure in polarized light. 

 Analyses of this rock gave 55*02 per cent, of silica, and the specific 

 gravity of the rock w r as found to be 2*82. 



The story told by the igneous and other rocks of Grange Hill 

 appears therefore to be this : — After the deposition of the unfossili- 

 ferous muds and sandy shales now found along the base of the 

 north-western slopes of the hill, the period of igneous activity 

 was heralded by a slowly creeping flow of andesite ; this made its 

 way along the sea-floor without producing any apparent chemical 

 alteration in the beds over which it crept. Soon, however, the ashes 

 which no doubt accompanied the outburst of lava settled down as 

 a thin deposit on its surface, enclosing here and there the animal 

 remains which are now found fossilized and give us proof that 

 all this occurred in Middle Bala times. The deposit of ashes was 

 then covered by another andesitic flow, and then the nature of the 

 lavas which crept over this portion of the sea-floor altered. They 

 became more decidedly basic and basaltic in character, the alteration 

 in chemical composition being accompanied by a darkening in 

 colour, and the more obvious and frequent occurrence of augites. 

 These rocks, too, are very vesicular in places, though the flow- 

 structure is not visible under the microscope as it is in the earlier 

 andesitic flows. The large felspars that floated along are, however, 

 all arranged with their long axes pointing more or less in one 

 direction, and testify to the fact that these rocks, too, crept along 

 the sea-floor and are true lavas. The upper surface of these flows 

 — no doubt very rugged and uneven — was then covered by a very 

 much coarser basalt, and as the latter crept along it appears to 

 have broken off pieces of the underlying flows and incorporated 

 them with itself. This flow also was vesicular, and appears to be 

 covered at one locality by a second flow of andesite. After this 

 igneous action ceased, and the deposition of mud and sands once 

 more occurred. But in places to which mud and sand were not 

 brought limestones were laid down, and finally the whole was 

 covered by the sandy beds which now form the grits that occur 

 south-east of the Chair. 



