On the supposed Pre- Cambrian Rocks of St. David's. 367 



more especially of certain fine tuffs and shales, which have passed into 

 the condition- of fine silky unctuous hydro-mica-schists or sericite- 

 schists. A series of microscopic slices was described which showed 

 that the original clastic structure of the beds remains quite distinct, 

 though an abundant development of fine flakes of a hydrous mica 

 has taken place. This structure more particularly characterizes the 

 fine parts of the volcanic group ; but it occurs also on various hori- 

 zons in the groups above the conglomerate, thus linking the whole 

 as one great continuous series of deposits. The author connected it 

 with the plication of the district, and pointed out the great interest 

 attaching to these fine schistose bands as revealing some of the 

 incipient stages of the same process that had changed wide regions 

 of sedimentary strata into crystalline schists. 



4. The Granite, Quartz- Porphyries, and accompanying Meta- 

 morpliism. — 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 gradation from a true granite into spherulitic 

 quartz-porphyry. The quartz-porphyries of St. David's (described 

 by Mr. Davies, 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 intermediate 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 intense 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. Examined microscopically, 

 the adinole is found to have acquired a microcrystalline structure, 

 nests of quartz and orthoclase and porphyritic 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 percentage 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 Yolcanic group, which 



