298 A. G-EIKIE ON THE SUPPOSED 



is that already referred to (p. 295) as No. 2 in the series of the 

 volcanic group. A specimen, taken from near Pen-jr-foel, is exter- 

 nally dirty-green, compact, and tolerably homogeneous, but presenting 

 distinct evidence of its clastic character (Plate IX. fig. 1). Under 

 the microscope it is found to be composed mainly of lapilli of a rock 

 somewhat different from any other which I have met with either 

 in the tuffs or among the interbedded or intrusive sheets. This 

 rock is marked by the abundance and freshness of its plagioclase 

 (an unusual feature in the volcanic group of St. David's); by the large, 

 well-defined crystals (one of which measured 0*022 inch by 0*0125 

 inch) of augite ; by large crystals replaced by viridite, but having 

 the external form of olivine ; by the absence or scantiness of any 

 base or ground-mass ; and, in many of the lapilli, by the abundance 

 of spherical cells, either empty or filled up as an^gdules with decom- 

 position-products. These spherical vapour-vesicles, so characteristic 

 of the lapilli in many Palaeozoic volcanic vents, were found in one 

 fragment, where they were particularly abundant, to range from a 

 minimum of 0*0008 inch to a maximum of 0*0033 inch, with a mean 

 of about 0*0018 (Plate IX. fig. 2). The rock from which these 

 lapilli have been derived comes nearest to one of the diabases from 

 the same part of the district (which will afterwards be referred to), 

 but shows a closer approach to the basaJt rocks. 



Another interesting tuff is that of which the analysis (No. II.) has 

 already been given. It occurs not far from the horizon of the rock 

 just described. Under a low power it is seen to be composed 

 mainly of fragments of diabase like the rocks of Phosson and 

 Clegyr Poig. These fragments are sub angular, or irregular in 

 shape, and vary considerably in size. They are sometimes finely 

 cellular — the cavities, as in the rock just described, being spherical. 

 The plagioclase crystals in the diabase lapilli are everywhere con- 

 spicuous ; so also is the augite, which occurs in larger forms than in 

 the rock of Phosson or Clegyr Poig. Next in abundance to these basic 

 fragments are rounded or subangnlar pieces of felsite. These weather 

 out in conspicuous grey rough projections on the exposed face of the 

 rock ; under the microscope they are seen to consist of fine granular 

 felsite, which shows a ground-mass remaining dark between crossed 

 nicols,but with luminous points and filaments, and an occasional sphe- 

 rulite giving the usual cross in polarized light. Lapilli of an older tuff 

 may here and there be detected. A few angular and subangular 

 grains of quartz are scattered through the rock. The lapilli appear 

 to be bound together by a finely granular dirty-green substance. 

 The presence of the quartz grains and of the felsite lapilli must 

 raise the proportion of silica, which no doubt, but for these admix- 

 tures, would have been a good deal less than 48*11 per cent., the 

 amount determined by Mr. "Wilson's analysis. 



As a type of the felsitic tuffs, I may refer to the rock already 

 described as No. Y. of the foregoing analyses. It is composed mainly 

 of fragments of various felsites, many of which show good fluxion- 

 structure. Large, and usually broken, crystals of orthoclase are 

 dispersed among the other ingredients. Here and there a fragment 



