Vol. 51.] THE COTJNTET ABOUND FISHGTJAKD. 187 



round the felspar-ciystals sometimes maintains not only the 

 crystalline form and faces of the crystal which it edges or termi- 

 nates, but even the twinning, and that many of the independent 

 groups of fibres merely differ from the ' incipient varioles ' by 

 possessing the crystallographic outlines of a felspar-crystal instead 

 of a spherical shape and radial structure. We can in fact see 

 felspar-crystals in the process of being built up of fibrous microlites ; 

 and I would ascribe their period of formation, together with that of 

 the ' incipient varioles,' to the general consolidation-period of the 

 rock. It is difficult, if not impossible, to suppose that they were 

 formed by a process of secondary devitrification posterior to the 

 solidification of the rock. 



There is a local variety of the Gam Fechan rock of considerable 

 interest. In it we find the sole instance of micropegmatite. The 

 groundmass in this rock [230] is composed of a granophyric 

 intergrowth of quartz and felspar, as in an ordinary acid granophyre. 

 The micropegmatite fills up the interstices between the lath-shaped 

 twinned felspar-crystals and the rhombic pyroxenes, and seems to 

 have been left as a sort of mother-liquor after these minerals had 

 crystallized out of the original magma. It reminds us of some of 

 the Carrock Fell rocks. The rhombic pyroxene (a kind of bastite) 

 is pleochroic, and fibrous in appearance ; and there is a large amount 

 of greenish serpentinous material present, as a decomposition - 

 product. Original clear quartz-grains are abundant. The specific 

 gravity of this rock is 2*67. 



The greater acidity of all these associated rocks which their 

 mineralogical composition indicates, may be due to the fusion and 

 incorporation of some of the acid lavas which they traverse. None 

 of these Fishguard variolitic rocks have the ' pseudocrystallites ' of 

 the Continental examples. 



There is a rock of great interest occurring in a field near Caerlem 

 Farm, on the north side of Garn Fawr, showing many features in 

 common with these Garn Fawr and Garn Fechan rocks. The 

 specimen from which the slide [262] was prepared forms a narrow 

 selvage to the boss of rock whence a previously-mentioned variolitic 

 rock [263] was obtained. Its high specific gravity at once arrests 

 attention, being 3*05, while that of the adjacent rock [263] is 

 only 2*93 and those of the tachylytic varieties [101] and [287] are 

 only 2-68 and 2*70 respectively 



The microscopic examination reveals other peculiarities, in the 

 presence of numerous spherical or elliptical amygdaloids and of 

 perlitic cracks. Some of these amygdaloids are lined with clear 

 quartz-grains, forming a zone of nearly constant breadth (that is, 

 about g diam. of amygdaloid) : the centre of these is filled with 

 fibrous radiating groups of chlorite ; others are entirely occupied 

 by quartz ; others again have a hollow centre. Epidote in irregular 

 patches or crystals is seen in the central or outer quartz-zone of 

 some. Others have an outer continuous zone, or a central kernel, of 

 a dichroic mineral with a high index of refraction ; this mineral 



