CHAP. X 
ST. DAVID'S 
149 
SiOa 
AI 0 O 3 
Fe^Oa 1 FeO 
MnO 
CaO 
MgO 
KoO 
Na *>0 
H-jO and 
Loss on 
Ignition. 
Total 
Specific 
Gravity. 
IV. 
80-59 
11-29 
0-28 T-41 
trace 
0-52 
0-95 
2-98 
0-72 
1-96 
100-70 
2-55 
V. 
73-42 
12-09 
0-91 3-13 
0-25 
2-94 
1-12 
1-67 
3-88 
1-28 
100-69 
2-74 
VI. 
72-63 
16-23 
2-70 0-48 
0-18 
1-36 
3-35 
0-15 
3-04 
100-12 
IV. Greenish felsitie breccia, Clegyr Hill ; angular fragments of various felsites in a greenish 
base. Analysis by Mr. J. S. Grant Wilson. 
V. Grey granular felsitie tulf, Bridge over Allan River north Irom St. David’s Board Schools. 
Analysis by Mr. Wilson. 
VI. Pale pinkish-white, finely schistose tuff — a characteristic sample of the “ Porth-lisky 
schists.” Analysis by Prof. Renard. 
Many varieties of texture can be traced among the tuffs, from coarse 
breccias or agglomerates, with blocks a yard or more in length, to fine 
schistose mudstones or sericitic schists. One of the most remarkable of the 
finer kinds, found near Pen-y-foel, is externally dirty-green, compact and 
tolerably homogeneous, hut with distinct evidence of its clastic character. 
Under the microscope it is found to be composed mainly of lapilli of a 
peculiar rock, which is characterized 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 measrired 0-022 inch by 0-0125 
inch) of augite ; by large crystals replaced by green decomposition-products, 
fiut having the external form of olivine ; by the absence or scantiness ot 
nny base or groundmass ; and, in many of the lapilli, by. the abundance of 
spherical cells, either empty or filled up as ainygdales with decomposition- 
products. These spherical vapour-vesicles, so characteristic of the basic or 
palagonitic lapilli in many Palteozoic 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. 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 sliows a closer approach to basalt rocks. 
Anotlier interesting tuff is that of which the analysis (No. II.) has 
fieen 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 
fiiabase like the rocks of Ehosson and Clegyr Poig. These fragments are 
subangular, or irregular in shape, and vary considerably in size. They are 
sometimes finely cellular — the cavities, as in the case just referred to, being 
spherical. The plagioclase crystals in the diabase-lapilli are everywhere 
conspicuous ; so also is the augite, wliich occiu-s in larger forms than in the 
I'ock of Ehosson or Clegyr Foig. Next in abundance to these basic frag- 
ments are roimded or subangukir pieces of felsite. These weather out in 
conspicuous grej^ rough projections on the exposed face of the rock ; under 
tbe microscope they are seen to consist of fine granular felsite, which shows 
