part 1] IGNEOUS AND ASSOCIATED ROCKS OF LLANWRTYD. 37 



regard to their structure, but the composition of the felspar makes 

 it unsuitable. It is true that felspar-microlites do occur in the 

 interspaces, but they seem to be due to the devitrification of an 

 interstitial glass, a view supported by the occurrence of patches 

 ■of a true microlitic spilite, differing markedly in texture from the 

 ■mass of the rock. 



Most of the felspar appears to be albite giving extinction-angles 

 in symmetrically- cut sections ranging from 10° to 16°, and having 

 a refractive index below that of balsam. Carlsbad twinning is 

 ■common, and certain laths giving straight extinction are probably 

 orthoclase. The felspar is, on the whole, fresh, although signs of 

 incipient alteration to a kaolin-like substance are not wanting. 



Filling the spaces between the felspars is a brownish murky 

 substance, which presumably arises from the decomposition of in- 

 terstitial glass. In some cases it appears to be aggregated into 

 spots, and it is also associated with granules of carbonates, impart- 

 ing to them a dusty appearance. 



Iron-ores, both primary (magnetite) and secondary (pyrites), 

 occur. 



No unaltered ferromagnesian mineral occurs in these rocks. 

 Certain chloritic patches fringed with iron-ore are probably pseudo- 

 morphs after some basic mineral. The iron-ore seems to have 

 separated from the chlorite, whence it may be inferred that the 

 original mineral had a high iron-content. It may be pointed out, 

 however, that neither hypersthene nor even amblystegite contains 

 enough iron to yield the excess which this implies, assuming that 

 the chlorite is intermediate in composition between amesite and 

 serpentine. The selvage of iron-ore is in general regular, and does 

 not vary much in width with the size of the chlorite-area. These 

 areas show two varieties of the mineral : the central parts are 

 occupied by a chlorite of fibrous nature, sub-radiate in arrange- 

 ment and possessing a relatively high birefringence. Adjacent 

 to the iron -ore rim is a zone of chlorite which is nearly isotropic : 

 this zone may represent an area from which the surplus iron has 

 been extracted. 



Vesicles are of various kinds. Some are filled with fibrous 

 chlorite, others, especially the larger, with calcite. Many, how- 

 ever, have a narrow border of chlorite, the interior being filled with 

 calcite, as is the normal arrangement in the Cornish spilites. Still 

 others are filled in the main with calcite, which encloses rounded 

 aggregates of chlorite. The presence of the latter suggests that 

 the post-volcanic periods of chloritic and calcitic deposition were 

 not always distinct and successive, but may have alternated and 

 overlapped. In a few cases, a rim of secondary water-clear albite 

 intervenes between the margin of the vesicle and the chlorite. 



A further point of interest as regards the vesicles is the common 

 •occurrence of an encircling zone of dark spilite. The latter differs 

 from normal spilite in containing fewer microlites, and in its 

 more advanced stage of decomposition. The relation of the dark 



