Yol. 54. J ON THE BALA & OTHEK EOCKS OP LAMBAY. 143 



band of augite-andesite. The rock, when fresh, is of a dark-green 

 colour ; small green felspars are with diflSculty to be made out, the 

 most obvious constituents of the rock being large black augites, 

 measuring as much as 6 mm. in length, and showing well- 

 marked cleavage ; the groundmass of the rock is fine-grained. 

 The microscope shows a groundmass of felspar-microlites, augite- 

 granules, magnetite, haematite, and chlorite ; in it are small porphy- 

 ritic felspars, now entirely replaced, and large augites. 



Going east- south-east from Raven's Eock, the last exposure of 

 this augite-andesite is close by a wall running north and south, 

 after which no exposures are to be seen for some distance. Almost 

 due east of the last exposure, however, near the wall, occurs a small 

 area of exposures, most of which are in a rock with a fine purple 

 groundmass containing bright yellow-green, rather small, porphyritic 

 felspars and large black augites, sometimes 9 mm. long. Other 

 exposures are in agreen-and-purplerock with a fine-grained ground- 

 mass, containing a few small porphyritic felspars and numerous 

 vesicles. 



About 450 yards due west of the summit of Flint Eock occurs a 

 small exposure of a greenish rock with a fine groundmass, no 

 distinctly porphyritic felspars, but with large black augites, some- 

 times 7 mm. long. This rock resembles markedly the augite- 

 andesites of Lambay Head and Raven's Bock ; but, as the ground 

 between Flint Kock and Knockbane is much covered with drift, it is 

 impossible to tell whether there is any extensive occurrence of the 

 rock between the two hills. 



Though augites occur in some of the other andesites, they are 

 found very seldom, and the above-mentioned type of rock, containing 

 abundant augites, was found nowhere else on the island. 



The remaining andesites call for very little remark. They are 

 generally of a very normal type of fine-grained andesite ; porphy- 

 ritic constituents are rarely visible in a hand-specimen, but under 

 the microscope they are all seen to contain porphyritic plagioclase- 

 felspars, now much replaced, and frequently the twinning is wholly 

 obliterated. Flow-structure is to be seen at times in the groundmass, 

 which is composed of small felspar-microlites and decomposition- 

 products, such as quartz, calcite, chlorite, and epidote, while either 

 magnetite, haematite, or ilmenite is generally present. A far from 

 uncommon porphyritic constituent of these rocks is apatite, which 

 occurs in the usual needles, giving straight extinction, while bastite- 

 pseudomorphs after hypersthene occur in the Kiln Point andesite. 



The alteration that has gone on in these rocks is very great, and the 

 resulting formation of secondary minerals most marked. In some 

 cases the production of epidote and quartz has been the result 

 following on the destruction of the porphyritic felspars ; in other 

 cases calcite has come in, and more or less spherical masses of 

 calcite occur (see fig. 3, p. 144), the concentric shells of calcite being 

 separated by shells of haematite, while in other cases chlorite occurs 

 abundantly. 



