268 
Mr. Weaver on the 
§ 169. It remains for me to consider only the connexion of the 
limestone with trap and porphyry. 
Croghan hill, near Philipstown in the King’s county, (see Plate 
6. No. 2.) is a protruding mass of trap rocks, rising about 500 
feet above the surrounding country, distinctly supporting on its 
north-western and south-western sides the floetz limestone, which in 
the former quarter dips 60° toward the north-west, and in the latter 
25® toward the same point, but acquires a higher elevation in re- 
ceding from the trap. 
The body of Croghan hill consists of trap conglomerate, which 
is more or less exposed on every side, and exhibits tabular masses 
dipping toward the north-west ; in the southern part under so high 
an angle as 80°, but in the northern of 60° only. Low down on 
the north-eastern side, where the hill is rocky and precipitous, ap- 
pears a hard, firm, compact rock of a bluish grey or pale lavender 
blue colour, arranged also in tabular masses, and dipping likewise 
60° toward the north-west ; and at the bottom of the precipice, the 
rock acquires a darker colour, being greyish black. This is an intimate 
mixture of hornblende and felspar, containing small disseminated 
particles of hornblende, calcareous spar, qu artz, and iron pyrites. The 
grey or blue rock, on the* other hand, is mixed with shades of muddy 
pink ; and seems to be an intimate mixture of compact felspar and 
carbonate of lime, of considerable tenacity, with a splintery or flat 
conchoidal fracture and translucent edges. It contains frequently 
disseminated particles of calcareous spar, of hornblende, and of 
iron pyrites ; and sometimes acquires an amygdaloidal character,, 
the portions of calcareous spar and hornblende assuming a roundish 
form. This general diffusion of calcareous matter through the 
rock, renders it very easy of decomposition near the surface ; and 
the carbonate of lime being gradually withdrawn by weathering, ai 
