208 



NATURE 



[October 17, 1912 



state their views and face discussion, which was often 

 very lively and always full of vigorous earnestness. 

 Then wc had a commodious and quiet meeting-room, 

 and the President (Dr. B. N. Peach) was a host in 

 himself. .As Dr. Heim said, in seconding a vote of 

 thanks to him : "When 1 look in front I always see 

 the sun," and the vivacious countenance and stirring 

 enthusiasm of Dr. Peach were an incentive to all 

 present to rise to their best. 



.After the delivery of the President's address on 

 "The Relation between the Cambrian Faunas of Scot- 

 land and North America," which was a timely and 

 most valuable deliverance giving the evidence for the 

 land connection between North America and North- 

 west Europe in Cambrian times, a lecture was de- 

 livered by Dr. T. J. Jehu on the local geology. After 

 a brief reference to the exposures of the Highland 

 Boundary series in Forfar and Kincardine, and to the 

 recent discoveries of rocks of Downtonian age near 

 Stonehaven, the lecturer dealt with the groups of rocks 

 found in the area : — (i) The Highland metamorphic 

 rocks; (2) the Old Red Standstone; (3) the Car- 

 boniferous system of Fife. 



(i) The pi'ominent series of metamorphic rocks com- 

 prises the Dunkeld slates, schistose slates and 

 micaceous grits included in the Ben Ledi group. 

 With the schists are associated the " Green Beds " 

 of the Geological Survey and certain basic intrusions 

 mapped as hornblende schists. 



(2) Two divisions of the Old Red Sandstone are 

 represented, Lower and Upper, separated by a marked 

 unconformity. They consist of two sedimentary 

 groups separated by the great volcanic series of the 

 Sidlaws and the Ochils. The distribution and charac- 

 teristics of the volcanic beds were described, and the 

 probable positions of the vents from which the 

 materials were ejected were indicated. The Lower 

 Old Red Sandstone rocks were folded and denuded 

 before the deposition of the LTpper Beds, which pass 

 conformably up into the Carboniferous System in Fife. 



(3) The Carboniferous rocks of Fife consist of the 

 Calciferous Sandstones at the base, overlaid by the 

 Carboniferous Limestone series, which are overlaid 

 by Millstone Grit and Coal Measures. Special atten- 

 tion was directed to the splendid development of work- 

 able coal-seams in the middle group of the Car- 

 boniferous Limestone series, as well as in the Lower 

 Coal Measures of the district. The remarkable diver- 

 sity of igneous rocks and the great development of 

 volcanic rocks on the coast of Fife were described, and 

 were subsequently displayed to the members by a field- 

 e.xcursion to Elie. 



During the meeting various problems connected with 

 the Pre-Cambrian rocks of the Highlands and the High- 

 land border were thoroughly expounded and discussed. 



Mr. E. B. Bailey described the occurrence in eastern 

 Mull of a great breccia-formation with intercalated 

 rhyolitic lavas. The breccia consists of an unbedded 

 assemblage of blunted rocks and fragments of gneiss, 

 granophyre, gabbro and basalt, often associated with 

 rhyolitic ddbris of volcanic origin. The basalt lavas 

 of Mull have been violently folded into a series of 

 anticlines and synclines, and it is in one of these 

 synclines that the main outcrop of the breccia is pre- 

 served. It appears that the breccia is a thick layer 

 overlying the basalts and folded with them. Though 

 some parts of the breccia may be of volcanic origin, 

 Mr. Bailey suggests that the greater part has resulted 

 from erosion, which operated during the period of 

 upheaval of the ridges. 



Dr. J. S. Flett opened a discussion on the sequence 

 of volcanic rocks in Scotland in relation to the .Atlantic- 

 Pacific classification of Suess. He pointed out that 

 the recognition of two great families of igneous rocks, 

 the -Atlantic and the Pacific, and their relation to 



NO. 2242, VOL. go] 



certain types of earth-movement, which we owe to 

 Mr. Harker, constituted one of the greatest advances 

 in rational petrology. In Scotland the Carboniferous 

 volcanic rocks are typical Atlantic types, associated 

 with collapse and great faults, and the rocks of Lower 

 Old Sandstone age are characteristic of the Pacific 

 group (great crumbling). To the Atlantic group could 

 be added the Permian or late-Carboniferous rocks of 

 Ayrshire and East Fife, and the nepheline-basalts (pre- 

 sumably Tertiary) of Caithness with their associated 

 camptonites and monchequites. The Tertiary volcanic 

 rocks of the Hebrides and the abundant north-west 

 dykes were also ascribed to movements of Atlantic 

 type. The remaining rocks of Scotland Dr. Flett 

 desired to assign to an independent, intermediate 

 group. They are characterised by pillow-lavas and 

 are not connected with movements either of the Pacific 

 or the Atlantic kind, and may thus be placed in a 

 special family. 



Dr. Flett's address was followed by a vigorous 

 discussion, in which considerable criticism was 

 expressed of the terms Atlantic and Pacific, and 

 especially of their application to t}'pes of rocks in 

 Scotland ; but Dr. Flett pointed out that some terms 

 must be used, and these were convenient, but he was 

 not pledged to them in any way. Dr. J. W. Evans 

 (London) thought that Dr. Flett had made out a 

 primd facie case, so far as the Scotch volcanic rocks 

 were concerned. He believed that the first differentia- 

 tion of igneous rocks was a result of the separation 

 of the original magma, on cooling, into two fluids, 

 one consisting mainh' of water, silica, alumina and 

 alkalies, the other made up chiefly of magnesia and 

 ferrous oxide with sufficient alumina and silica to 

 form anorthite and the ferric minerals. Subsequent 

 differentiation was principally due to crystallisation. 

 In folded areas the segregation took place at a con- 

 siderable depth, while the magma still retained its 

 original aqueous contents. The primary differentiation 

 was therefore comparatively complete, and the normal 

 or " Pacific " series of rocks was ultimately formed. 

 In districts of plateau faulting, on the other hand, 

 the differentiation occurred nearer the surface and 

 under conditions which facilitated the loss of water, 

 so that the separation was imperfect, the basic portion 

 retaining more silica and a considerable amount of 

 alkalies, and subsequent segregation by crystallisation 

 gave rise to the alkali or "Atlantic" series. He also 

 suggested that the occurrence of sodium with the basic 

 rocks might be due to the presence of an unusual 

 amount of chlorine, which separated out with the basic 

 constituents and brought the sodium with it. 



Dr. Tempest .Anderson did not agree with the sugges- 

 tion that pillow-lavas had been formed in deep water, 

 and gave instances which he had seen of their formation 

 by lavas flowing into shallow waters, and apparently 

 due to sudden cooling by the waves. Mr. G. W. Tyrrell 

 (Glasgow) much preferred the terms " alkalic " and 

 "calcic" for Dr. Flett's main divisions. The differ- 

 ences were mainly chemical and mineralogical, not 

 geographical. Many group names cover a wide diver- 

 sity of types, and we appreciate the great value of 

 Dr. Flett's recognition of mono- and poly-phyletic 

 rocks, that is, types highly characteristic of one group 

 only and types to bo found in all the main divisions of 

 igneous rocks. Dr. Hatch pointed out Ihe advantage 

 of a division which recognises a well-defined suite of 

 rocks in which the alkali and calc-alkali felspars are 

 developed in equal proportions, a series to which 

 Brogger has given the name Monzonite series, and 

 hoped that such a classification would be generally 

 adopted. 



Mr. G. Barrow gave a valuable paper on the older 

 granite in Lower Dee Side, in which he showed that 

 in place of forming large coherent masses, it was 



