HARDWICKE'S SCIENCE-GOSSIP. 



i35 



of olivine-diabase. Their augite is remarkably 

 abundant and fresh, and they contain scattered larger, 

 well-formed, as well as imperfect crystals of olivine, 

 generally in the form of hsematitic pseudomorphs. 

 No instance was observed of a siliceous lava having 

 been erupted at the surface. The felsitic fragments 

 in the tuffs must have been derived from the explosion 

 of lavas that do not seem to have flowed out above 

 ground. It was pointed out that this fact is exactly 

 paralleled in the case of the volcanic group of the 

 Lower Old Red Sandstone in the Pentland Hills. 

 In relation to the quartz-conglomerate, allusion was 

 made to the constant recurrence of such conglomerates 

 in the series of geological formations, and to the fact 

 that they do not necessarily mark unconformability 

 or the natural base of groups of sedimentary rocks. 



2. Geological Structure of the District. — It was 

 shown that the rocks have been folded into an isocline 

 or inverted anticline, so that in one-half of the plica- 

 tion the dip of the strata is reversed. The groups 

 above mentioned are found in their proper order on 

 both sides of the axis which runs through the volcanic 

 group. The granite has risen irregularly through the 

 eastern limb of the isocline. Small faults may occur 

 here and there along the edge of the granite, but they 

 do not in any way affect the general structure. 



3. The Foliation of the District. — There has been 

 extensively developed at St. David's a fine foliation 

 of particular kinds of rock, more especially of certain 

 fine tuffs and shales, which have passed into the con- 

 dition 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 particu- 

 larly characterises the fine parts of the volcanic group, 

 but it occurs also on various horizons 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 accom- 

 panying Metamorphism. — The petrographical cha- 

 racters 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 microscopi- 

 cally, the adinole is found to have acquired a 

 micro-crystalline 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"8oofsoda, 

 indicating possibly the formation of albite. The 

 author deferred generalising 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. Both 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 

 volcanic vents in that region in an early part of the 

 Lower Cambrian period. The volcanic accumula- 

 tions 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 a later 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. The last episode was that of the diabase 

 dykes, which, crowded together in the granite, 

 suggest that the granite boss stands on an old line of 

 weakness and of escape for eruptive material from the 

 interior. 



In the Discussion which followed, Dr. Hicks stated 

 that since the last meeting he had revisited the 

 district with Professor Hughes and ten excellent 

 trained observers from Cambridge. This examina- 

 tion confirmed, to the fullest extent, the views 



