282 Reports and Proceedings— 
the same process that had changed wide regions of sedimentary strata 
into crystalline schists. 
4. The Granite, Quartz-Porphyries, and accompanying Metamorphism. 
—The petrographical characters of these eruptive rocks were described, 
and their perfect analogy to the familiar granites and elvans of other 
districts was pointed out. Specimens were shown illustrating the 
eradation from a true granite into spherulitic quartz-porphyry. The 
quartz-porphyries of St. David’s (described by Mr. Davies and Dr. 
Hicks, and others) exhibit spherulitic structure in an exceptionally 
perfect manner. Between the felso-spherulites the base is thoroughly 
micro-crystalline and not felsitic. The rocks belong to a group inter- 
mediate between granites and felsites. They occur in bosses, elvans, or 
‘dykes round the granite, cutting through all horizons of the volcanic 
group, and approaching, if they do not actually intersect, the quartz- 
conglomerate. ‘The metamorphism associated with the granites and 
porphyries is best seen near the latter. It consists chiefly in the in- 
tense induration of certain bands of rock which have been converted 
into flinty aggregates (adinole). The alteration takes place usually 
along the bedding, which is nearly vertical; but veins of the same 
siliceous material ramify across the stratification of the shales. Ex- 
amined microscopically, the adinole is found to have acquired a 
micro-crystalline structure, nests of quartz and orthoclase and porphy- 
ritic crystals and plagioclase having been developed, together with 
fine veins and filaments of crystalline quartz. These veins are here 
and there crowded with approximately parallel partitions of liquid 
inclusions showing freely moving bubbles. An analysis of a portion 
of the adinole, made for the author by M. Renard, shows the per- 
centage of silica to be 78°62 with 5°80 of soda, indicating possibly the 
formation of albite. The author deferred generalizing on the question 
of the metamorphism he described, but pointed out that a further 
study of the St. David’s rocks could hardly fail to throw important 
light on the theory of metamorphism. 
5. The Diabase Dykes and Sheets—These are the latest rocks at 
St. David’s, as they traverse all the others. Their macroscopic and 
microscopic characters were described, and allusion was made to the 
perfect fluxion-structure found in many of the dykes. 
The paper closed with a summary of the geological history of St. 
David’s. The earliest records are those of the Volcanic group, which 
show the existence of voleanic vents in that region in an early part of 
the Lower Cambrian period. The volcanic accumulations were covered 
conformably by the Conglomerate and succeeding Cambrian groups ; 
but the same kind of tuffs continued to be ejected after the deposition 
of the Conglomerate. At alater time this thick conformable succession 
of beds was plicated, and underwent a partial metamorphism, whereby 
some of the fine tuffs and shales were converted into sericite-schists. 
Subsequently a mass of granite rose through one side of the fold, 
accompanied by elvans of spherulitic quartz-porphyry, whereby a 
second, different, and feebler kind of metamorphism was induced. 
‘he last episode was that of the diabase dykes, which, crowded to- 
gether in the granite, suggest that the granite boss stands on an old 
line of weakness and of escape for eruptive material from the interior. 
