^88 I'ROF. T. G. BONNEY AND MISS C. A. KAISTN ON [May 1 899, 



rhombic), olivine, and iron-oxide, apparently magnetite. The last 

 varies in quantity, but is never abundant: the first on the whole 

 dominates, occurring in large crystalline grains, lustre-mottled by 

 olivine, and is in various stages of change. The least altered 

 is ordinary bastite, vith the characteristic cleavage, faintly pleo- 

 cliroic,^ exhibiting fairly bright polarization-tints and straight 

 extinction. In places it is of a pale dull-green, and consists of a 

 matted mass of steatitic fibres, which, with crossed nicols, only 

 show here and there specks or gleams of whitish or brownish light. 

 This might be regarded as another variety of enstatite inter- 

 crystallized with the former: but, after careful study, we came 

 to the conclusion that it is more probably only a further stage of 

 decomposition, though sometimes the change from the one to the 

 other seems abru])t. The olivine occurs in grains of two sizes, 

 the larger independent, the smaller included (PI. XXIII, fig. 5). 

 Commonly it is serpentinized in the usual way, but sometimes it 

 is bordered by a zone of minute actinolite, growing like grass 

 on the edge of the bastite ; sometimes it is pierced by larger 

 fiakes of the same, similaily related, and in one example these grow 

 ])Mrallel with the fibres of the enclosing bastite, and entirely take 

 the place of the small olivine- grain, w^hich is then only obvious 

 from a difference in tint, and an oblique extinction when the 

 ]]icols are crossed ; sometimes the olivine is wholly replaced by 

 matted actinolite. Microliths of the same occasionally pierce the 

 enstatite, in which (though rarely) they are extensively developed. 

 A little iron-staining occasionally may be seen, and in one a crystal 

 or two of (?)sphene. 



These structures are also exhibited by rocks in situ, especially by 

 those in a craglet south of Cerig-moelion Farm.^ On microscopic 

 examination the included olivine-grains are ftequently seen to have 

 a radial border, which may exhibit certain difi'erences. Sometimes 

 the border is very narrow, and the fibres forming it rather resemble 

 serpentine ; they are set somewhat obliquely, but seem to be con- 

 tinuous with the central mineral.^ Here the inner part shows the 

 parallel vein -like structure already described, and may be derived 

 from olivine ; but even in these the possibility of enstatite must be 

 admitted. The enclosing mineral is generally an augite or diallage, 

 sometimes partly changed into pale hornblende. In one slice a 

 curious modification is found : instead of the fringe around the 

 included grains, clear secondary hornblende, frequently wedge- 

 shaped, has grown out at the edges of the original ])yroxeue in the 

 way which has been often described in a diabase. 



^ Straw-colour with vibrations perpendicular to the easy cleavage, and sap- 

 green with parallel vibrations, both pale. 



^ Also to a oertain extent in the cliff south of Rhyd-bont. 

 • ** A marginal zone was defccribt-d as a ' reaction-rim' by Tornebohtn (Neues 

 Jahrb. 1«77, p. 383) and G. H. Williams (Am. Joiirn. Sci. ser. 3, vol. xxx, 18815, 

 p. 35) ; and a similar structure was attributed by W. S. Bayley to the result of 

 change in interstitial augite, Am. Journ. Sci. ser. 3, vol. xliii (1892) p. 515. In 

 the examples described by tbe^e authors the rim is formed between olivine and 

 felspar ; in the Anglesey rocks, between a pyroxene and another pyroxene or 

 olivine. In any case, the fctructure can only be compared indirectly with the 

 variolee. . 



