188 MK. E. E. COWPEB KEED ON THE GEOLOGY OE [May 1895, 



also occurs in minute prisms or detached patches in the outer quartz- 

 zone of some of the amygdaloids. It gives a sea-green colour parallel 

 to the length of the crystal and to the shorter diameter of the 

 nicol, and is colourless in a position at right angles to this. The 

 extinction is straight. From these characters I believe it is one of 

 the epidotes. 



The amygdaloids represent original vesicles. After the absorption, 

 condensation, or escape of the gas the cavities thus formed became 

 more or less infilled with mineral matter. The dark line which 

 surrounds most of these amygdaloids seems due to some corrosive 

 action of the gas on the groundmass. The perlitic cracks in this 

 rock point to its original glassy condition ; some of them have been 

 enlarged and filled in with quartz or epidote, and magnetite in 

 granules has collected along them. 



This slide [262] also shows two corroded felspar-crystals with 

 crowds of dark brownish microlites diverging from their borders, 

 while the surrounding groundmass is devoid of such structures. 

 There are also a few spongy-looking ' clots ' of crystallites and 

 granules which represent feebly the well-developed ' nodules ' of 

 a rock from Penrhiw Farm, near Goodwick, which will now be 

 described. 



This Penrhiw rock is another variety of the glassy intrusives, 

 and it may be appropriately called the ' nodular ' variety. In 

 the hand-specimen this rock has not the least appearance of an 

 intrusive mass, but rather that of a nodular rhyolite ; it is of a 

 glaucous-green colour, with circular white spots, each of which has a 

 violet centre ; these are not, however, the ' nodules/ but only 

 amygdaloids of quartz, etc. ; the ' nodules ' themselves are irregu- 

 larly oval patches, | inch or so in length, and of an opaque whitish 

 appearance. The fracture of this rock is subconchoidal, and re- 

 sembles that of a fine-grained felsite. 



Under the microscope we notice many points of resemblance to 

 the tachylytic variety : for instance, the parallel augite-crystallites, 

 and the linear rows of granules ; but there are also larger spherical 

 grains of a semi-opaque greenish-brown mineral, and groups of 

 crowded parallel minute crystallites. Yariations occur in the relative 

 abundance of the different elements. The ' nodules ' are rather 

 irregular ovals, measuring | inch or less in length, and have a 

 darkish, almost opaque, spongy-looking outer portion, the pores and 

 interstices of which are filled with secondary quartz ; the central part 

 is of a pale brownish-yellow colour, with fine dark rods and fibres 

 (crystallites), similar to those in the groundmass, either radiating 

 towards the centre or in feathery groups. A skeleton felspar-crystal 

 is seen in the centre of one ' nodule,' but as a rule no definite 

 felspathic elements are recognizable. The ' nodules ' are frequently 

 marked off plainly from the groundmass by concentric cracks or a 

 dark granular thin coat, and are more or less irregular in shape. 

 The dark granular outer coat or border is particularly well-marked 

 around the nodules in one slide [40]. 



