Vol. 51.] THE COUNTRY AROUND FISHGUARD. 181 



grains and crystals of magnetite are so thickly crowded together as 

 to render it almost opaque under the microscope ; and in another 

 rock, from an old quarry near Manorowen Mill, similar characters 

 are found. These important differences in composition must be 

 ascribed partly to a segregation or concentration of the constituents 

 of these minerals in certain portions of the unconsolidated and un- 

 crystallized magma, and partly to successive intrusions of varying 

 composition. While it was yet fluid a kind of lateral secretion and 

 interchange of material must have gone on, for there is no sign of 

 movement subsequent to the formation of the crystals of the minerals 

 except perhaps in the case of the magnetite. 



The felspars belong for the most part to the oligoclase-andesine 

 set. As a rule, the felspar-crystals are of one generation only, and 

 present the usual lath-shaped forms in section ; but two generations 

 are observable in some cases [259], [43], [109], in which the earlier 

 generation consists of large, well-formed twinned crystals, and the 

 later of small, interlaced, lath-shaped crystals. 



In many cases the latter contain granular cores or inclusions, 

 especially when the rock belongs to the ' slaggy ' type ; and 

 skeleton-crystals are very common in this type, together with 

 feathery aggregates of felspathic fibres [101], [257], [227], [263], 

 etc. (see also infra). 



In the majority of cases the felspars have clearly preceded the 

 other minerals (with the exception of the iron oxides) in crystal- 

 lizing out from the original magma ; and their outlines are moulded 

 by the other constituents. In some cases the predominant mineral 

 in the rock is felspar, as on Cam Ingle [304], [308], and [350]. 



The augite is usually of later date than the felspar, but some- 

 times belongs to nearly the same period of crystallization, as in the 

 coarse-grained, gabbro-like rocks of Ciliau and Cam Gowel [3], 

 [107], in which almost or partly idiomorphic crystals occur. 1 Good 

 crystals also are found in a rock from Castle Point, Fishguard [62]. 

 Large ophitic augites are seen in the Henner School mass, and also 

 in dolerites from Fishguard Harbour [72], and elsewhere [108], 

 [241]. A peculiarly minute ophitic structure is shown by the augite 

 in a rock [267] from the neighbourhood of Drittwg, Strumble Head. 



A rather exceptional type, in which the augite occurs in abundant, 

 large, fresh, crystalline grains [310], but not ophitically, though it is 

 rather later than the felspar, leads us on to the ' granulitic ' type 

 of Judd, 2 in which the augite is in a granular condition. This is 

 largely represented in our area [252], [282], and [43]. 



A dark grey rock from the eastern side of Porth Sychan shows 

 fairly well-formed crystals of augite, in addition to granules of this 

 mineral. Perhaps the latter may be due to a shattering of some of 

 the crystals by internal movements in the unconsolidated magma. 



1 See Harkcr, 'The Bala Volcanic Series of Caernarvonshire,' pp. 81, 82. 



2 Quart. Journ. Geol. Soc. vol. xlii. (1886) p. 49. 



