Vol. 54.] ON THE BALA <fc OTHEE ROCKS OF LAMBAY. 145 



The other example of this structure in a fine andesite is in a 

 rock some 200 yards south-west of Kiln Point. 



In these three slides, though the alteration of the groundmass 

 has gone on to so great an extent, the porphyritic felspars are 

 remarkable on account of their freshness. 



Here and there the andesites are amygdaloidal, and occasionally 

 as much as half the rock consists of amygdules ; this is especially 

 well seen in a rock obtained from a spot about halfway between 

 Heath Hill and Bishop's Bay. 



The inclusions noted as occurring at the mouth of Thorn chase 

 Valley and on Lambay Head are also to be seen along the cliffs 

 between Bishop's Bay and Sank Island Bay, and on Sunk Island 

 itself, while the island of Carrickdorrish also shows them all over 

 its surface. They were very possibly blown on to the surface of 

 the andesites, and gradually incorporated in them, or they may 

 have been portions of the rocks which formed the vents through 

 which the andesites rose to the surface. 



(c) The Coarse Porphyrite. 



The last type of rock remaining for description is the coarsely 

 porphyritic rock, well known as the 'Lambay porphyry,' and 

 described by Von Lasaulx both macro- and microscopically.^ This 

 writer mentions that it has been described as an amphibolite-green- 

 stone, as a quartzless orthoclase-porphyry, and as a diabase-porphyry, 

 and pronounces in favour of the last name. 



From the map (PI. IX) it will be seen that the coarse rock occurs 

 in very many places on the island, but in only a few does it extend 

 over a large area, the rock being more usually found in small 

 unconnected patches. 



The coarsest variety was found in a small exposure on the seaward 

 face of the point forming the south-western edge of Sunk Island 

 Bay. Here the groundmass is green in colour and very compact, 

 while large green platy felspars, sometimes as much as 23 mm. broad, 

 are present in great abundance, showing repeated twinning. Calcite 

 occurs in little rounded masses, possibly filling vesicles or replacing 

 augites. More usually, however, the porphyritic felspars are 

 smaller ; thus in the exposure between the Coastguard Station and 

 Scotch Point the groundmass is also green and compact, but the 

 felspars are not more than 11 mm. broad ; here augite is a fairly 

 abundant porphyritic mineral, crystals of this mineral measuring as 

 much as 6 mm. in length. In the mass occurring round Trinity Well 

 the porphyrite has a compact purple groundmass ; the porphyritic 

 felspars are numerous, and run up to 12 mm. in length, being green 

 and platy like those described above. Amygdules are fairly common 

 in this rock, being formed of calcite or of calcite and chlorite. 



The long strip of coarse rock exposed along the base of the hills 



^ Tschenn. Min. u. Petrogr. Mitth. 1878. 

 a.J. G.S. No. 213. L 



