490 



NATURE 



[September 21, 1893 



Hi par Hi has been shown by M. Levy to be a pDtent cause in 

 the formation of certain banded gneisses. 



Will the various agencies to which reference has been made 

 explain all the phenomena of the crystalline schists and gneisses ? 

 I do not think that the present state of our knowledge justifies 

 us in answering this question in the affirmative. Those who 

 are working on these rocks frequently have brought under their 

 notice specimens about the origin of which they are not able to 

 speak with any degree of confidence. Sometimes a flood of 

 light is suddenly thrown on a group of doubtful rocks by the 

 recognition of a character which gives unmistakable indications 

 of their mode of origin. Thus, some of the fine-grained quartz- 

 felspathic rocks associated with the crystalline schists of the 

 Central Highlands are proved to have been originally sands like 

 those of Hampstead Heath by the presence in them of narrow 

 bands rich in zircon, rutile, and the other heavy minerals which 

 are so constantly present in the finer-grained arenaceous deposits 

 of all ages. Such pleasant surprises as the recognition of a 

 character like this increase oir confidence in the theory which 

 endeavours to explain the pist by reference to the present, and 

 refuses to admit the necessity of believing in the existence of 

 rocks formed under physical conditions different from those 

 which now prevail simply because there are some whose origin 

 is still involved in mystery. 



A crystalline schist has been aptly compared to a palimpsest. 

 Historical records of priceless value have often been obscured by 

 the superposition of later writings ; so it is with the records of 

 the rocks. In the case of the schists, the original characters 

 have been so modified by folding, faulting, deformation, crystal- 

 lisation, and segregation that they have often become unrecog- 

 nisable. Bat when the associated rocks have the composition 

 of sediments we need have no hesitation in attributing the 

 handed structure in some way to stratification, provided we 

 clearly recognise that the order of succession and the relative 

 thicknesses of the original beds cannot be ascertained by apply- 

 ing the principles which are valid in comparatively undisturbed 

 regions. 



In studying the crystalline schist; nothing, perhaps, strikes 

 one more forcibly than the evidence of crystal-building in solid 

 rocks. Chiastolite, staurolite, andalusite, garnet, albite, cor- 

 dierite, micas of various kinds, and many other minerals have 

 clearly been developed without anything like fusion having 

 taken place. Traces of previous movements may not unfre- 

 quently be found in the arrangement of the inclusions, while 

 the minerals themselves show no signs of deformation. Facts 

 of this kind, when they occur, clearly indicate that the crystal- 

 lisation was subsequent to the mechanical action. Nevertheless, 

 it is probable that both phenomena were closely related, though 

 not in all cases as cause and effect. The intrusion of large 

 masses of plutonic rock often marks the close of a period of 

 folding. This is well illustrated by the relation of granite to 

 the surrounding rocks in the Lake District, the Southern 

 Uplands of Scotland, and the West of England. Those of the 

 two first-mentioned localities are post- Silurian and pre-Carboni- 

 ferous, those of the last-mentioned locality are post-Carbonifer- 

 ous and pre-Permian ; one set followed the Caledonian ' fold- 

 ing, the other set followed the Hercynian folding. That the 

 intrusion of these granites was subsequent to the main move- 

 ments which produced the folding and cleavage is proved by 

 the fact that the mechanical structures may often be recognised 

 in the crystalline contact-rocks, although the individual minerals 

 have not been strained or broken. In many other respects the 

 rocks produced by so-called contact-metamorphism resemble 

 those found in certain areas of crystalline schist. Many of the 

 most characteristic minerals are common to the two sets of 

 rocks, and so also are many structures. The cipolins and asso- 

 ciated rocks of schistose regions have many points of resem- 

 blance to the crystalline limestones and " kalksilicathornfels " 

 produced by contact-metamophism.^ 



These facts make it highly probable that, by studying the 

 metamorphic action surrounding plutonic masses, we may gain 

 an insight into the causes which have produced the crystalline 

 schists of sedimentary origin ; just as, by studying the intrusive 

 masses themselves and noting the tendency to petrographical 

 differentiation, especially at the margins, we may gain an insight 

 into the causes which have produced the gneisses of igneous 



' This term is employed in the sense in which it is used by Suess and 

 Bertrand. 



» H. Rosenbusch, " Zur Auffassung des Grundgebirges," Nenes Jahr. f. 

 Miner., Bd. II. (1889) p. 8. 



origin.' In the districts to which reference has been made the 

 igneous material came from below into a region where the rocks 

 had been rendered tolerably rigid. Differential movement was 

 not taking place in these rocks when the intrusion occurred. 

 Consider what must happen if the folding stresses 

 operate on the zone separating the sedimentary rocks from the 

 underlying source of igneous material. Intrusion must then 

 take place during interstitial nnvement, fluxion structures will 

 be produced in the more or less differentiated igneous m.igmas, 

 the sediments will be injected and impregnated with igneous 

 material, and thermo metamorphism will be produced on a 

 regional scale. The origin of gneisses and schists, in my 

 opinion, is to be sought for in a combination of the thermal and 

 dynamic agencies which may be reasonably supposed to operate 

 in the deeper zones of the earth's crust. If this view be correct 

 it is not improbable that we may have crystalline schists and 

 gneisses of post-Silurian age in the North-West of Europe 

 formed during the Caledonian folding, others in Central 

 Europe of post- Devonian age due to the Hercynian folding,and 

 yet others in Southern Europe of post-Cretaceous age produced 

 in connection with the Alpine folding.- Bat if the existence of 

 such schists should ultimately be established it will still pro- 

 bably remain true that rocks of this character are in most cases 

 of pre-Cambrian age. May not this be due to the fact, sug- 

 gested by a consideration of the biological evidence, that the 

 time covered by our fosisliferous records is but a small fi action 

 of that during which the present physical conditions have 

 remained practically constant? 



The good old British ship " Uniformity," built by Hutton and 

 refitted byLyell, has won so many glorious victories in the past, 

 and appears still to be in such excellent fighting trim, that I see 

 no reason why she should haul down her colours either to 

 " Catastrophe " or "Evolution." Instead, therefore, of acced« 

 ing to the request to " hurry up " we make a demand for more 

 time. The early stages of the planet's history may form a 

 legitimate subject for the speculations of mathematical 

 physicists, but there seems good reason to believe that they 

 lie beyond the ken of those geologists who concern themselves 

 only with the records of the rocks. 



In this address I have ventured to express my views on 

 certain disputed theoretical questions, and I must not coriclude 

 without a word of caution. The fact is, I attach very little 

 importance to my own opinions, at least on doubtful questions 

 connected with the origin of the crystalline schists ; but, f.s you 

 have done me the honour to accept me as your President, I 

 thought you might like to know my present attitude of mind 

 towards some of the unsolved problems of geology. There is 

 still room for legitimate difference of opinion on many of the 

 subjects to which I have referred. Meanwhile, we cannot do 

 better than remember the words with which one of our great 

 living masters recently concluded an article on a controversial 

 subject : " Let us continue our work and remain friends." 



NO. T247, VOL. 48I 



SECTION D. 



BIOLOGY. 



Opening Address bv Rev. H. B. Tristram, M.A., LL.D., 

 D.D., F. R.S., President of the Section. 



It is difficuU for the mind to grasp the advance in biological 

 science (I use the term biology in its wide etymological, not its 

 recently restricted sense) which has taken place since I first 

 attended the meetings of the British Association, some forty 

 years ago. In those days, the now familiar expressions of 



1 G. Barrow, " On an Intrusion of Muscovite-biotite-?neiss in the South 

 Eastern Hiehlands of Scotland, &c." Quart. Joutn. Geol. Soc, vol. xli». 

 (1893), p. 330. . -J 



- Some geologists maintain that this is the case, others deny it. See H. 

 Reusch. '• Die fossilienfiihrenden krystallinischen Schiefer von Bergen in. 

 Norwegen," Leipzig (1883) J. Lehmann, " Uber die Entstehung der 

 allkryslallinischen Schiefergesteine, mit besonderer Bezugnahme auf des 

 siichsische Granulitgebirge, Erzgebirge, Fichtelgebirge. und bairisch- 

 biihmische Grenzgebirge," Bonn, (1884): T. G. Bonney. several papers on 

 the Alps, and especially "t)n the Crystalline Schists and their Relation l» 

 the Mesozoic Rocks of the Lepontine Alps," Quart. Joiirit. G'.ol Sal., vol. 

 xlvi. (1890). p. 188; A. Heim. contribution ti the discussion on the last 

 paper; C. W. Giimbel, " Geognostische Beschreibung des K. B.ayern and 

 " Gnindziige Her Geologic," Kissel (1888-1892). . • t l 



Although it isconvenient to speak of the three typesoff.jding which havt 

 so largely influenced the structure of the European continent as if eacB 

 belonged to a definite period, it is important to remember that this is not 

 strictly true. The movements were prolonged ; they probably crept slJWiy 

 over the surface of tke lithosphere, as did the zones of sedimentati >n, so that 

 th'jse of the same type are not in all places strictly contemporaneous. 



