182 ME. F. E. COWPEE EEED ON THE GEOLOGY OF [May 1 895, 



Iron oxides occur both as magnetite and ilmenite. The latter is 

 found in the coarse-grained rocks, and usually as skeleton-crystals 

 with the characteristic decomposition-product leucoxene [107], [24], 

 [60], and [82]. 



Magnetite is not generally abundant. When found it is in the 

 form of minute rods, grains, and crystals [10]. But in some basalts, 

 etc., it occurs in considerable quantity, as in the Cam Hendy rock 

 [293],. and others [326], [259], and [237]. 



Apatite is rare, but when found has the usual form of slender 

 prisms [28]. Interstitial matter is present in the basaltic, tachy- 

 lytic, and variolitic rocks ; and almost invariably we find, as we should 

 expect, that its presence in any abundance indicates the proximity of 

 the margin of the intrusion [286], [273], and [275]. 



Variations in the texture of the rock may be observed even in 

 the same slide [288], and probably point to the later injection of 

 matter of a slightly different composition into the unsolidified mass, 

 or to the inclusion of earlier consolidated portions, or to the differentia- 

 tion in situ of the molten magma. Flow-structure is even indicated 

 in some cases by the parallelism of the felspar- crystals [339]. 



The secondary minerals call for some special notice. The usual 

 products of the decomposition of the original minerals are present 

 in most of the rocks, and of these secondary minerals chlorite is the 

 commonest : it may occur either as structureless pale green patches, 

 or as groups of radiating fibres lining cavities or filling up the inter- 

 stices between the other minerals. In a few cases the pale struc- 

 tureless patches contain circular clear spots, the origin of which is 

 doubtful. With crossed nicols the chlorite is almost isotropic, but 

 these spots are seen to consist of quartz or chalcedony, [19] and 

 [106] : sometimes the spots have a dark semi-opaque centre, but 

 only in one instance [247] is each spot a spherulite giving a perfect 

 black cross with crossed nicols. 1 



Some of the greenish decomposition-minerals appear to be of a 

 serpentinous nature, but olivine has not been detected in any of the 

 rocks. 



Epidote is common as a secondary product, and is found as small 

 grains or crystals. Calcite, in large crystalline plates with well- 

 marked cleavage, is especially abundant in the sills on the western 

 side of Fishguard Harbour ; and dolomite also occurs here. The 

 sills at this locality have suffered not only much chemical alteration, 

 but also considerable mechanical movement and crushing. The 

 planes of the cleavage thus set up are marked by shining black 

 flakes of a chloritoid mineral, [59] and [30]. Calcite occupies large 

 patches of the rock and forms irregular pseudamygdules, so that 

 when it has subsequently been removed by solution these weathered- 

 out cavities give to the rock a pseudo-scoriaceous appearance. 



The associated cleaved tuffs of vesicular lavas, lapilli, etc., of 



1 Mr. S. H. Reynolds, M.A., has recently pointed out to me similar struc- 

 tures in some Kildare basalts ; and I have met with them also in dolerites from 

 Co. Waterford. 



