part 1] IGNEOUS AND ASSOCIATED EOCKS OP LLANWRTYD. 41 



identical with the ' chequer-albite ' of F. Becke ; this variety is also 

 found in the Devon quartz-keratophyre tuffs. 1 The effect under 

 •crossed nicols is due to the splitting-up into a number of small 

 rectangular patches, half of which extinguish simultaneously. 

 Becke supposed the structure to be secondary, 2 and, in support of 

 this surmise, we may note that in these rocks ' chequering ' some- 

 times affects the periphery of a crystal only. 



Indications of a micropegmatitic fringe can be seen round 

 certain of the felspars — a feature referable to the devitrification of 

 the original matrix prior to silicitication. 



In addition to crystals of quartz and felspar there are bombs of 

 fine-grained spilitic material, as well as pumiceous fragments the 

 vesicles of which have been pulled out into long tubes and filled 

 with secondary quartz. Among the smaller recognizable consti- 

 tuents are crystals of zircon, often associated with iron-ore. The 

 ' Lower Acid ' tuffs of the Cader-Idris district also show this 

 feature. 



It thus appears that these rocks compare closely with those 

 described as quartz-keratophyre tuffs in other districts, the resem- 

 blance extending even to points of minor detail. 



Banded ashes occur, as already stated (p. 22), on the eastern 

 side of Garn Dwad, following upon the coarser keratophyric ashes 

 just described, There is some sign of passage between them and 

 the rocks below. 



A typical example shows, under the microscope, a succession of 

 bands, not very clearly defined, ranging in thickness from 3 mm. 

 to less than 1 mm. The darker bands seemingly owe their colour 

 to magnetite-dust, which defines the bedding-planes to some 

 extent. 



Generally speaking, the rock is a fine-grained aggregate of quartz 

 and felspar showing local chloritic and limonitic staining. Calcite 

 is abundantly distributed, especially in the paler bands. The 

 quartz is angular, and full of inclusions. The felspars retain in 

 general a lath-shape, and, although it is not possible to measure 

 extinction-angles in such small fragments, the refractive index is 

 below that of balsam in most cases. Thus these rocks agree with 

 their associates in being predominantly albitic. 



The fine-grained facies of the Upper Ashes, to which the term 

 horn stone or halleflinta is applied, appears to compare in 

 many respects with the ' china-stone ashes ' which are so widely 

 distributed in the Llanvirn Beds, as at Hope Dingle, Abereiddy 

 Bay, and in the Dolgelly district. The white weathering is very 

 characteristic. Under the microscope, the rocks are seen to be 

 almost identical with the Ynys-Castell Ashes, as described by 

 Prof. A. H. Cox. 



1 Mem. Geol. Surv. 1913, p. 58. 



2 Denkschr. K. Akad. Wissensch. Wien, vol. lxxv (1913) p. 128. 



