NEIGHBOURHOOD OF SARN, CAERNARVONSHIRE. 445 



a verj'' pale green for the direction at right angles. A further stage 

 of alteration produces an indistinct brownish scaly mass, which 

 finally becomes isotropic with the separation of magnetite-dust. The 

 chief felspar forms rectangular crystals, with fine twin-lamellation 

 and low extinction-angles, agreeing with those of oligoclase. Ortho- 

 claso occurs in large plates of later formation than the plagioclase ; 

 both felspars show kaolinization, beginning from the centre of the 

 crystals. Quartz is abundantly present, often in composite grains, 

 moulding the felspars ; it contains many minute fluid-cavities of 

 irregular shapes, with spontaneously moving bubbles. 



The granite has apparently undergone some degree of secondary 

 change connected with the operation of mechanical stresses. Some 

 of the fluid-inclusions in the quartz, as remarked by Mr. Tawney, 

 occur in lines which pass from grain to grain. These lines of inclu- 

 sions seem to be connected with roughly parallel cracks which 

 traverse quartz and felspar alike. There are other cracks marked 

 by a finely granular mixture of felspar and quartz, and sealed by 

 epidote and irony staining ; this appearance may perhaps be com- 

 pared with the " Mortelstructur'^ of Tornebohm. The gliding-planes 

 sometimes seen in the mica are probably another secondary pheno- 

 menon. 



The granite and gneissic granite of Meillionydd have a character 

 in some respects differing from the normal type. The constituents 

 are the same as before, but their arrangement is anomalous, the 

 usual order of consolidation being partly reversed. The felspars, 

 chiefly a finely laminated acid plagioclase as before, are sometimes 

 older, sometimes newer than the bulk of the quartz ; a bipyramidal 

 crystal of the latter mineral is occasionally seen included in felspar. 

 The biotite is the latest-formed mineral. It moulds the earlier pro- 

 ducts of consolidation and fills the interstices between them, even 

 sending out little tongues into cracks and irregular inlets in the 

 quartz. A portion of the latter mineral, however, is sometimes of 

 more recent formation than the biotite, and may even be posterior 

 to the consolidation of the rock. 



The occasional gneissic aspect of the granite at Meillionydd seems 

 to have no necessary connexion with the peculiarities described. 

 It consists in a local banded structure, with partial separation of 

 the constituent minerals, seen only in large specimens. It is most 

 probably to be referred to a certain amount of differential move- 

 ment of the mass during the process of solidification, and cannot be 

 due to any subsequent crushing of the solid granite. Little seg- 

 regatory nests, rich in brown mica^ occur in the rock, and these 

 show no evidence of distortion. 



The granite of Cefn-amwlch, Meillionydd, &c., is named syenite 

 by the Geological Survey, a name which does not seem properly 

 applicable ; and it is stated that the " greenstone " of Mynydd-y- 

 Ehiw " passes into " the " syenite." This idea of a passage, unlikely 

 in itself, does not appear to be borne out by the evidence in the 

 field. 



Dr. Hicks, in 1879, put forward the view that the granitic mass, 



