Yol. 55.] SERPENTINE AND ASSOCIATED ROCKS IN ANGLESEY. 281 



(2) Banded Serpentine. 

 The serpentine has in places a banded structure, perhaps best 

 shown in connexion with some of the vario]itic rock described in tho 

 next parngraph. In other examples, and even in some of the varioliiic 

 forms, the rock is characterized rather by an irregular streaking than 

 by a definite banding, both however being probably due to fluxion. 

 In one example from behind Cerig-moelion Quarry the structure is 

 partly schistose, but here probably pressure acted on a rock already 

 banded.^ 



(3) Yariolitic Serpentine. 



Small spheroidal bodies are usually weathered out on exposed 

 surfaces of this rock, varying from about J to about ^ inch across (or 

 from the size of a millet-seed to that of a pea). In many spherules 

 the centre is of paler colour, or even whitish, and sometimes it is 

 replaced by a somewhat irregular cavity. Microscopic examination 

 suggests that a carbonate has been rather abundantly present, but 

 in these examples has weathered out. The spherules fcrm patches, 

 in which they are irregularly grouped : though here and there 

 distinct, they are generally crowded, and the crowding some- 

 times becomes greater, until they form a confused aggregate 

 in which no distinct si)herules can be detected" In some 

 bosses where no spherules can be recognized clearly, a confused 

 structure suggests the incipient stage of a similar formation.^ Tho 

 spherules in a patch may be small or large; and usually different 

 sizes are adjacent, or arc closely associated and even mixed. 

 Very frequently the patches take the form of streaks or bands, as 

 in a spherulitic obsidian. The rock in such cases usually shows 

 a streaking in the intermediate parts, and thus a iiuxion-structuro 

 is distinctly suggested. 



The colour and appearance of the rock differ somewhat in the 

 different examples : — 



(a) South of Grraig-fawr. — The boss on the beach is roughly 

 about 20 feet in length, and is prolonged by small knobs. Tho 

 rock is of a rather bright green with a little dark mottling. It 

 includes paler green spherules, commonly of the size of a millet- 

 seed. These sometimes form irregular patches, but often lenticular 

 streaks or bands. The rock is very brittle, its fracture in places 

 almost resembling that of obsidian, and having, when fresh, a 

 glassy lustre ; it is translucent, and resembles noble serpentine. 

 On the weathered duller surfaces the spherules are very pale or 

 whitish, and the patches of them sometimes project as slightly 

 mammillated knobs. The broken spherule often consists only of a 



^ The microscope-slice shows some streaky serpentine yielding bluish polari- 

 zation-tints resembling that sometimes derived from enstatite. See Quart. 

 Journ. Geol. Soc. vol. xxxvii (1881) p. 45. 



- This recalls a similar transition olten seen in variolitic diabase. Seo 

 G. A. J. Cole & J. W. Gregory, Quart. Journ. Geol. Soc. vol. xhi (1860) 

 p. 313; J. W. Gregory, ibid. vol. xlvii (1&91) p. 48; C. A. Raisin, ibid. 

 YoL xlix (1893) pp. ir)4, 155 & pi. i, fig. 5. 



^ Sonielimes tijcy can be recognized macroscopically, vs^here microscopic 

 examinalion reveals only a confused aggregate. 



