190 ME. E. K. COWPER REED ON THE GEOLOGY OE [May 1895, 



two dark, thread-like perlitic cracks run close to each other in a 

 parallel or sinuous manner, and have only a clear, transparent, broad 

 line or narrow zone between them. In some cases the walls of the 

 crack have gaped apart more widely, so that the dark line is thick 

 and granular, representing the infilled crack. The perlitic cracks 

 have in fact acted as channels, along the sides of which alteration of 

 the surrounding rock has taken place. 



There is another rock from Penrhiw which completes the strange 

 trio from this spot. It occurs as part of the same mass as the 

 ' nodular ' and perlitic varieties, and is the most distant of the 

 three from the outer margin of the intrusive mass. In the field it 

 is a pale greenish-grey rock, with minute black specks scattered 

 through it. Under the microscope in ordinary light it shows [73] 

 a clear, transparent groundmass, with feathery groups of felspar- 

 fibres, parallel groups of clear spindle-shaped crystallites, flecks, 

 grains, irregular little rods and threads of a semi-opaque, dark 

 brownish substance, small circular patches of quartz, chlorite, etc., 

 and long prismatic crystals with a high index of refraction and 

 ragged ends which seem to tail out into spindle-shaped crystallites. 

 These crystals with crossed nicols give bright polarization-colours, 

 and extinguish at 40° to their length. They appear to belong to 

 some pyroxene. Lines of flow are very apparent in this rock. 



A basalt, with peculiar characters not found in any other in the 

 district, occurs at Cam Hendy, near Ciliau. The groundmass [293] 

 is composed of fine feathery bunches and radial tufts of felspathic 

 crystallites, with an immense quantity of interstitial magnetite in 

 grains and dust, which gives the rock a very dark colour and high 

 specific gravity. It is also remarkable for the number of vesicles of 

 all sizes and shapes, which are filled with serpentinous minerals, 

 chlorite, and some epidote. Skeleton felspar-crystals and elongated 

 slender felspars with an axial core of inclusions are also scattered 

 throughout the groundmass. The rock somewhat resembles a vesicular 

 ferruginous basalt north of Forth Oer, Lleyn peninsula, described 

 and figured by Miss Raisin. 1 It belongs to the great northern 

 intrusive mass of Strumble Head, but is a very distinct local 

 variety, and is another indication of the composite nature of this 

 huge intrusion. 



Vesicles. — Nearly all the igneous intrusions are more or less 

 vesicular in parts. But the vesicles have not all the same origin ; 

 for there are, firstly, the amygdaloids which fill up the cavities pre- 

 viously occupied by gas-bubbles, and these have usually a spherical 

 shape. Secondly, there are the amygdaloids which occupy spaces 

 that have been formed through the removal by solution of portions 

 of the groundmass or of crystals of felspar, etc. These might be called 

 * pseudomorphic amygdaloids.' They are present in many of the 

 rocks [282], [109], [78], and [10], and are usually distinguishable 

 from the amygdaloids after gas-bubbles by their irregular shape. 



Certain dark patches of interstitial matter, which in some cases 



1 Quart. Journ. Geol. Soc. vol. xlix. (1893) p. 164 & pi. i. fig. 2. 



