136 MR. GARDINER AND PROF. REYNOLDS ON THE [May I909, 



Of accessory minerals, iron-ores are represented with remarkable 

 frequency ; ilmenite occurs occasionally (273), a general red stain 

 due to haematite is very common, and at a number of points (4, 

 16, 68, 90) magnetite is present in some abundance. Occasionally 

 apatite is rather plentiful (194, 281, 284). Small irregular vesicles 

 are sometimes present filled with calcite, or with serpentine 

 (187), or with chlorite and chalcedony, the latter mineral being 

 sometimes spherulitic. 



(b) The Intrusive Andesitic Rocks. 1 

 Three types of these rocks are recognizable : — 



1. Specimens collected from various points along the dyke west 

 of Mount Partry are all dark compact rocks, showing little in a 

 hand-specimen but occasional small dark amygdules. In section 

 the main part of the rock is seen in every case to be composed of 

 needles of plagioclase, which, from the straight or almost straight 

 extinction, are no doubt to be referred to oligoclase. These needles, 

 which are so disposed as to show flow-structure, have interspersed 

 with them a relatively smaller number of larger plagioclase-pheno- 

 crysts in a more altered state. One slide (130) shows brown patches, 

 which are probably altered augites, but ferromagnesian minerals 

 are very scanty. Small vesicles are always present, generally 

 occupied by a chloritic mineral. The rock exposed at (3), the point 

 where the dyke is cut by the Bohaun road, contains vesicles the 

 central parts of which are occupied by quartz, then follows a layer of 

 chlorite, while the marginal parts of the vesicle are occupied by epidote 

 and calcite (see PI. V, fig. 4). A good deal of epidote is scattered 

 throughout this rock. One specimen (283), taken from near the 

 northern end of the dyke, is much more uniformly vesicular than 

 the remainder; and the little patch of andesite (292), near the 

 margin of the red felsite north of Gortanalderg, is of similar type, 

 being crowded with small chlorite-filled vesicles. Very little iron- 

 ore occurs in any of these rocks. In many respects, these rocks seem 

 to be closely allied to those described from the St. David's district 

 by Mr. J. V. Elsden 2 as lime-bostonites. 



2. The small andesitic intrusions in the Arenig rocks from the 



1 [Subsequently to the reading of this paper, Dr. J. S. Flett kindly examined 

 our sections of these rocks, and reported on them as follows: — 'Although 

 rocks like these have been described as andesites, andesitic dolerites, etc, in 

 several of our memoirs and elsewhere, they are not good andesites. Their 

 felspars are all albite-oligoclase and oligoclase, and they contain no femic 

 minerals, only obscure pseudomorphs after pyroxene. I have compared them 

 with the type-slides of msenites, and they are not a bit like them. I should 

 call them spilites myself, as their characters, mainly negative I admit, are 

 those of this group. They have the essential features of the Mid-Devonian lavas 

 of the Plymouth area, though they are not exactly like any of the groups of 

 spilites I am familiar with. If they contained more alkali-felspar, they would 

 closely resemble some keratophyres. I cannot get over the belief that they are 

 "pillow-lavas".'] 



2 Quart. Journ. Geol. Soc. vol. lxi (1905) pp. 594 et seqq. 



