May 3. 1883] 



NA TURE 



19 



made for the author by M. Renard of Brussels, and Mr. J. S. 

 Grant Wilson of the Geological Survey of Scotland. The tuffs 

 are partly basic, derived from the disruption of diabase lavas 

 (48 per cent, of silica), partly acid, from the destruction of 

 felsites (72 to 80 per cent, of silica). The microscopic structure 

 of the tuffs was described, and slides and drawings were ex- 

 hibited. The lavas are varieties of olivine-diabase. Their augite 

 is remarkably abundant and fresh, and they contain scattered 

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

 rally in the form of hcematitic pseudomorphs. No instance was 

 observed of a siliceous lava having been erupted at the surface. 

 The fel-itic fragments in the tuffs must have been derived from 

 the explosion of lavas that do not seem to have Bowed out above 

 ground. It was pointed out that this fact is exactly | aralleled 

 in the case of the volcanic group. of the Lower Old Red Sand- 

 stone in the Pentlai d 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 d ) not neces- 

 sarily mark unconformability or the natural base of groups of 

 sedimentary rocks. 



2. Geological Structure of the District. — It »a- shown that 

 the rocks have been folded into an isocline or inverted anticline, 

 so that in one-half of the plication 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 01 

 rock, 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 

 remain^ quite distinct, though an abundant development of fine 

 flakes of a hydrous mica has taken place. This structure more 

 particularly 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 con inuous 

 series of deposits. The author connected it with the plication 

 of the district, and pointed out the great interest ataching to 

 these fine schi-tose 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- 

 morphism. — The petrographical characters of these eruptive 

 rocks were described, and their perfect analogy to the faunl'ar 

 granites and elvans of other districts was pointed out. Speci- 

 mens were shown illustrating the gradation from a true granite 

 into spherulitic quirtz-p irohyry. 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 ftlso-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 horizot s of the volcanic 

 gn.up, 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 altera- 

 tion 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 micro-crystalline structure, 

 nests of quartz and orthoclase and porphyritic crystals of plagio- 

 clase having been developed, together with fine veins and fila- 

 ments of crystalline quartz. These veins are here and there 

 crowded with approximately parallel partitions of liquid inclu- 

 sions 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 7862 with 580 of soda, indicaing 

 pos-ibly the formation of albite. The author deferred genera- 

 lising on the question of the metamorphism he described, but 

 pointed out that a further study of ihe St. David's rocks could 

 hardly fail to throw important light on the theory of meta- 

 morphism. 



5. The Diabase Dykes and Sheets. — These are the latest rocks 

 at St. David's, as they traverse all the others. Their macro- 

 scopic 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 

 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 a later time this thick conformable succession of I eds 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 diaba-e 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. 



As the conclusions drawn by the author from his study of the 

 microscopic structure of the rocks of St. David's had been called 

 in question at the reading of the first part of the paper, he took 

 an opportunity before the reading of the second part to submit 

 a series of typical specimens and microscopic slides to Professors 

 Zirkel of Leipzig, Renard of Brussels, and Wichmann of Utrecht. 

 These observers amply sustained his deductions. M. Renard 

 came from Brussels to be present at the reading of the second 

 part, and in the course of the discussion stated that Professors 

 Zirkel, Wichmann, and himself had arrived at the following 

 conclusions regarding the rocks of St. David's : — 



I. The so-called " Dimetian " rock of St. David's is unques- 

 tionably a true grinite. 2. The quartz-porphyries are just such 

 rocks as might be expected to occur as apophyses of the granite, 

 and the specimens from Bryn-y Gam, Rock House, and St. 

 David's left no doubt on their minds that such is really their 

 origin. They cannot be confounded with rhyolitic lavas. 3. 

 The conglomerate from the granite-contact shows secondary 

 quartz between its pebbles. 4. The bands of fine tuff found 

 intercalated with, and on various horizons above, the con- 

 glomerate, consist of true tuff, and cannot have been derived 

 from the mere superficial waste of older vol anic rocks. 5. Fine 

 foliation is well developed am ng the strata above the con- 

 glomerate as w ell as in the volcanic group below. 



SOLAR PHYSICS 1 



TH E lecturer introduced his subject by draw ing attention to the 

 circumstance that the idea of the sun being an exceedingly hot 

 body was of very modern date, that both ancient and modern 

 writers up to the early portion of the present century attributed to 

 him a glorious and supernatural faculty of endowing us with light 

 and heat of the degree necessary for our wellbeing, whilst even 

 Sir William Herschel had attempted to find an explanation to 

 account for his idea that the body of the sun might be at a low 

 temperature, and inhabitable by beings similar to ourselves, 

 which he did in surrounding the inhabitable surface by a non- 

 conducting atmosphere — the penumbra — to separate it from the 

 scorching influence of the exterior photosphere. 



It was not till the views of Kant, the philosopher, had been 

 developed by Laplace, the as'ronomer, in his famous " Me- 

 canique Celeste," that the view gained ground that our central 

 orb was a mass of matter in a state of incandescence, represent- 

 ing such an enormous aggregate as to continue radiation into 

 space for an almost indefinite period of time. 



The lecturer illustrated by means of a diagram the fact that of 

 all the heat radiated away from the sun only 1/2,250,000,000 

 part could fall upon the surface of our earth, vegetation and 

 force of every kind being attributable to this radiation, whilst all 

 but this fractional proportion apparently went to waste. 



Recent developments of scientific research had enabled us to 

 know much more of the constitution of the sun and other 

 heavenly bodies than had formerly been possible. Comte says 

 in his " Positive Philosophy " (Martineau's translation of 1853) 

 that "amongst the things impossible for us ever to know was 

 that of telling what were the materials of which the sun was 

 1 Abstract of Lecture at the Royal Institution, by Sir William Siemens. 

 F.R.S.. April 27. 



