142 MESSES. C. I. GAEDII^EE & S. H. REYNOLDS [Feb. 1 898, 



(b) The Andesitic Rocks. 



As will be seen by reference to the map (PL IX), much the greater 

 part of the island is formed of rocks of an andesitic nature. In a 

 hand- specimen some of them show small porphyritic felspars, and 

 some are difficult to class, as they appear to be intermediate between 

 the andesites and the coarse porphyrite. 



At certain spots the andesites include angular blocks of igneous 

 rock, sometimes in large numbers, as on Lambay Head and in the 

 cliffs at the mouth of Thornchase Valley ; while at other spots fine 

 and coarse breccias occur among them, as above Freshwater Bay near 

 Gillap. The inclusions in the rock at the mouth of Thornchase 

 Valley are noted in the Survey Memoir. 



The andesites as a rule show no augites, but examples of 

 augite-andesite occnr. The rock at Kiln Point is a hypersthene- 

 andesite, the hypersthene being now replaced by bastite. One of 

 the inclusions in the andesite at Carrickdorrish shows numerous 

 porphyritic crystals ; these cannot be determined with certainty, 

 owing to the alteration which has gone on, but they are very 

 probably altered olivines. Some of the inclusions at the mouth of 

 Thornchase Valley show augites, though the enclosing rock does not 

 appear to have any. 



As shown in the map, the chief exposures of augite-andesite are 

 close to the top of Knockbane, close to Raven's Rock, west of Eliut 

 Rock, and on Lambay Head. 



The Lambay Head augite-andesite is a very well-marked rock 

 running along the crest of the Head towards Pilot's Hill, its large 

 augites being readily seen on its weathered surfaces. In a hand- 

 specimen this rock shows a dark green, somewhat horny-looking 

 groundmass, in which small felspars are sparingly scattered and 

 large porphj'ritic augites occur, the largest one seen measuring 

 9 mm. in length. A microscope- section of this rock shows a fairly 

 prominent, highly altered groundmass, in which are many short, 

 broad felspars, now entirely replaced. The porphyritic felspars are 

 also entirely converted into a quartz-mosaic, but the porphyritic 

 augites are very fresh, and occur in granular aggregations ; mag- 

 netite is present, while chlorite and epidote have come in as secondary 

 minerals. 



The augite-andesite which occurs close to the top of Knockbane 

 has a fine, compact, somewhat horny groundmass, purple in colour, 

 containing numerous ill-defined patches of a green colour (generally 

 rounded in outline), and small black patches are also to be seen. 

 The microscope shows the groundmass to be now formed of secondary 

 minerals, quartz, iron-ores, and calcite, while the porphyritic 

 felspars which it contains are also entirely altered. Augites are 

 represented by a brown decomposition-product, which here and there 

 encloses cores of the original undecomposed mineral. 



Running east-south-east from the steep cliff known as Raven's 

 Rock, and traceable for some 350 yards, is a third well-marked 



