part 1] IGNEOUS AND ASSOCIATED EOCKS OF LLANWETYD. 39 



(«) The first type comes from a horizon not far ahove the 

 spilites. It is a well-bedded rock intermediate in character between 

 the crystal -tuff s and the lithic tuffs, as defined by L. V. Pirsson. 1 

 It consists of a number of small fragments, many not exceeding 

 1 mm. in diameter, of an ill-defined felsitic character, mixed with 

 many felspar-crystals, complete or fragmental, and a few quartz- 

 grains. The fragments are in close juxtaposition, only a little 

 argillaceous material intervening. Some few are vesicular, and an 

 occasional spilitic fragment may be found. It is remarkable that 

 the underlying lavas have contributed so little to the rock, which 

 would appear to have been derived from a relatively acid magma. 



(b) The second type forms a fairly constant band at a horizon 

 just below the Ashy Limestone. It is a beautiful example of a 

 lithic tuff, as defined by Pirsson. It is chiefly composed of 

 fragments of spilite in close juxtaposition. The fragments were 

 apparently once glassy, and, as in the Lower Breccias already 

 described, the decomposition-products of the glass tend to obscure 

 the felspar-laths. 



Filling the angular spaces between the fragments is a some- 

 what remarkable substance, which appears at first sight to be a 

 vesicular calcite : that is, calcite-areas dotted over with small 

 chlorite-filled vesicles. This seems to be a case of the almost 

 complete calcification of a vesicular glass. 



The rock moreover carries small pisolitic bodies as described in 

 the spilite-breccias, and a fragment of an echinoderm-plate was 

 also found. 



(5) The Upper Ashes. 



These beds weather like typical ashes, and compare closely in 

 hand-specimens with the finer parts of the Cefn-Hir Ashes recently 

 described from North Wales. 2 Under the microscope, the rocks are 

 very similar to ashes from the Lower Acid Series of the same 

 authors. 3 



The rocks are of thoroughly acid composition, and consist of a 

 number of large crystals (some broken) of quartz and felspar, set 

 in a silicified matrix which may be described as felsitic. In places, 

 however, the allothigenous fragments of quartz and felspar in the 

 matrix are larger, and it is then possible to distinguish them from 

 secondary quartz-granules, and to recognize their angular and 

 unworn character and the clastic appearance which they impart to 

 the rock. This matrix must, before consolidation, have been of 

 the nature of an unsorted volcanic sand. 



Turning now to the larger constituents, we may note that, while 

 the quartz is commonly angular, this is not a universal feature, and 



1 Amer. Journ. Sci. ser. 4, vol. xl (1915) p. 193. 



2 A. H. Cox & A. K. Wells, Q. J. G. S. vol. lxxvi (1920-21) p. 278. 



3 Rep. Brit. Assoc. (Manchester, 1915) p. 424. 



