12 The Irish Naturalist. January 



occasional twinning, but a few apparently much altered 

 felspars also are just noticeable. 



The well-known Lambay porphyry, which is a porphyritic 

 type of andesite, occurs in several usually rather restricted 

 outcrops in various parts of the island. The outcrops are 

 usually fairly conspicuous by reason of the relatively superior 

 weathering qualities of this rock, which result in well-marked 

 hillocks and rock scarps. When fresh, the ground-mass is 

 of a dark green colour, in which the light-coloured felspar 

 shows up prominently. On the hill-slope east of the castle, 

 the ground -mass is purple in colour, owing to iron staining, 

 and the rock is very much jointed. It is rather unfortunate 

 that this makes the rock unsatisfactory for use as an orna- 

 mental stone, and only in one localit) r , just on the west of 

 Sunk Island Bay (Plate 6), does it appear possible to procure 

 specimens of this rock of a reasonable size and without flaws. 

 The out crops of this rock are indicated on the accompanying 

 map hy a special sign, as shown in the index. 



It occurs chiefly as dykes or sills in the andesite, and pro- 

 bably as an extruded mass at Flint Rock and Pilot's Hill, 

 where it appears to rest on a brecciated surface of an under- 

 lying andesite. Occasionally the porphyry is amygdaloidal, 

 and sometimes also contains fragments of pre-existing ande- 

 sites. 



In section, the ground-mass of the rock is a much-altered 

 granular crystalline aggregate of felspar and augite, with 

 secondary minerals, amongst which epidote, iron ore, pyrites, 

 sphene, and calcite are conspicuous. The porphyritic felspars 

 which characterise the rock are fairly basic, and possess 

 extinction angles which make them fall among the labradorite 

 felspar group. 



The remaining rocks are not very extensively developed, 

 and belong to the fragmental series known as volcanic ashes. 

 These consist of angular and sub-angular fragments of ande- 

 sites, and occasionally shales, of all sizes, from about one foot 

 to the finest dust. It is nearly impossible to decide in many 

 cases whether the fine-grained matrix of these rocks is really 

 of fragmental origin, or whether it represents an altered 

 compact igneous rock. The microscope gives practically no 

 information, and fails to distinguish between an altered 



