30 



Proceedings of the Royal Irish Academy. 



to any extent by contact with the igneous material, except for about an inch 

 from the junction, where its flinty character seems to be intensified. 



This is the most basic rock observed in the district. It consists for the 

 most part of a ground mass of small pink pyroxene crystals, with a little 

 brown interstitial glass. Many larger crystals of a pale pyroxene occur, and 

 pseudomorphs of olivine, in which no trace of the original mineral is seen ; 

 they can, however, be easily detected by their outline. The phenocrysts of 

 augite and altered olivine are set in a complex of augite microlites. Much 

 magnetite occurs in small grains (fig. (5). The rock is a limburgite or magma- 

 basalt with a specific gravity of 2 - S4. and bears a close resemblance to some 

 of the Scotch limburgites of Carboniferous Age, particularly the rock from 

 Chester Quany, Haddingtonshire, described by Dr. Hatch. 1 A section of 

 this rock was examined from the collection in the Eoyal College of Science, 

 Dublin. The Irish example diii'ers from this only in containing less glass, 

 its place being taken by augite microlites ; there is, moreover, no fresh 

 olivine, but otherwise the rocks appear to be identical. 



Fig. 6.— 20. Fig. 7.— x 36. 



Just outside Philipstown, and across the canal to the north-east, at Castle 

 Barnagh, a small knoll rises which is visible for some distance. It is formed 

 by an intrusion of igneous rock, which has been forced up through the lime- 

 stone. X o ash was found in its neighbourhood. It is a dark compact rock, 

 aud is so highly charged with carbonate of lime as to effervesce freely with 

 acid. In section it is seen to be crystalline, with numerous notched felspathic 

 areas which enclose crystals of augite and magnetite ; also numerous very 

 slender needle-like crystals which may possibly be apatite. The augite occurs 

 as good idiomorplric crystals, and also with a second generation of smaller 

 crystals in the matrix. Some olivine occurs as pseudomorphs in ealeite and 

 chlorite (fig. 7 . A feature of the rock is the quantity of mica it contains 



1 Dr. Hatch. " Lower Carboniferous 'X olcanic Rocks of East Lothian," Trans. Roy. 

 Edinburgh. 1892, 37. p. 116. 



