Oct. 5, 1882] 



NA TURE 



567 



by the conductivity, using the word "conductivity" (as above 

 explained) in such a sense as to include convection. The rate of 

 increase downwards must, therefore, be the same at all depths 

 at which this conductivity is the same. 



This reasoning applies to superposed strata at the same place, 

 and assumes them to be sufficiently regular in their arrangement 

 to ensure that the flow of heat shall be in parallel lines, not in 

 converging or diverging lines. 



5. If we have reason to believe that the flow of heat upwards 

 is nearly the same at all places, then the abo :e reasoning can 

 also be applied approximately to the comparison of one place 

 with another — that is to say, the rates of increase downwards, in 

 two masses of rock at two different places, must be approxi- 

 mately in the inverse ratio of their conductivities. In the cooling 

 of a heated sphere of heterogeneous compo-ition, the rates of 

 flow would at first be very unequal through different parts of 

 the surface, being most rapid through those portions of the 

 substance which conducted best ; but these portions would thus 

 be more rapidly drained of their heat than the other portions, 

 and thus their rates of flow would fall off more rapidly than the 

 rates of flow in the other portions. If the only differences in 

 the material were differences of conductivity, we might on this 

 account expect the outflow to be after a long time nearly the 

 same at all parts of the surface. But w hen we come to consider 

 differences of "thermal capacity per unit volume," it is clear 

 that with equal values of "diffusivity," that is of "conductivity 

 divided by thermal capacity of unit volume " in two places, say 

 in two adjacent sectors of the globe, there would be the >aue 

 distribution of temperatures in both, but not the same flow of 

 heat, this latter being greatest in the sector in which the capacity 

 and conductivity were greatest. 



Where we find, as in Mr. Deacon's observations at Bootle, 

 near Liverpool, and to a less marked degree in the observations 

 of Sir William Fairbairn and Mr. Gaiside, near Manchester, an 

 exceptionally slow rate of increase, without exceptionally good 

 conductivity, it is open to us to fall back on the explanation of 

 exceptionally small thermal capacity per unit volume in the 

 underlying region of the earth, perhaps at depths of from a few 

 miles to a few hundred miles. 



6. A question which was brought into consideration by Prof. 

 Hull, in connection with the great difference between the rate of 

 increase at Dukinfield and that at Rosebridge, is the effect of the 

 dip of the strata npon the vertical conduction of heat. Lami- 

 nated rocks conduct heat much better along the planes of lami- 

 nation than at right angles to them. If k x denote the conducti- 

 vity along, and k„ the conductivity normal to the planes of 

 lamination, and if these planes are inclined at an angle 6 to the 

 horizon, the number of feet per degree of increase downwards 

 correrponding to a given rate of outflow through the surface, 

 will be the same as if the flow were vertical with a vertical 

 conductivity : — 



£, sin* 8 + £ 2 cos" 8. 



Prof. Herschel finds about I "3 as the ratio of the two principal 

 conductivities in Loch Rannoch flagstone, and 1 'S75 as the ratio 

 in Festiniog sla'.e. 



The dip of the strata at Dukinfield is stated by Mr. Garside to 

 be 15°, and we have sin 2 15° = -07, cos 2 15° = 93. 



If we asume i t = 13 /?.,, as in the case of flagstone, we find 

 for the effective vertical conductivity £, (09 + '93) = 102 /■», so 

 that the number of feet per degree would only be increased by 

 2 per cent. 



It is not likely that the two conductivities in the strata at 

 Dukinfield are so unequal as even in the case of flagstone, so 

 that 2 per cent, is a high estimate of the effect of their dip on 

 the vertical rate of increase so far as pure conduction is con- 

 cerned. The effect of dip in promoting the percolation of 

 water is a distinct consideration, but the workings of the Dukin- 

 field mines are so dry that this action does not seem to be 

 important. 1 



(7b it ctnlinued.) 



METAMORPHIC ROCKS OF BERGEN 1 

 THE metamorphic rocks of the Bergen Peninsula in Norway 

 - 1 continue to attract the attention of Norwegian geologists, 

 and we have before us, as an addition to the well-known works 



1 Though the workings are dry [here is a large quantity of water in the 

 superincumbent strata. 



* Hans H. Reusch, " Silurfi. ssiler eg Pressede Konglc 

 genskifrene." — Universitetsprogramm forftrste Halvaar (i~~ 



of Naumann, Leopold von Bucb, Esmark, Keilhau, Kjerulf, 

 and Hjortdal, a new elaborate and interesting work by M. 

 Hans H. Reusch, which deals with the same subject. These recks 

 consist, as is known, of a variety of quartziferous talc-mica 

 schists, diorite, clay-slates, conglomerates, and strongly-developed 

 gneisses and granites. Various and very different opinions 

 have been expressed as to the origin of these rocks. The 

 researches of M. Reusch give a key to this question, as he has 

 discovered in the clay-slates, which seem to constitute the upper 

 part of these vertical strata, numerous fossils belonging to the 

 lower part of the Upper Silurian formations, namely Halysitts 

 eattmilaria and Cyathophyllum, changed into white calcareous 

 spar, a few tubular bjdies (presumably Syringophyllum organum), 

 some gasteropods (Murcliisonia or Siibulites ?) sou.e trilobites, 

 as Calymenc, also Phacopsox Dalmannitcs, and some brachiopr.ds. 

 The presence ot the-e fossils is the more interesting as the 

 whole series of schists was often considered as of igneous origin. 

 As to the gneisses and gneisso-granites of the peninsula, M. 

 Reusch has given great attention to their structure and to 

 the remarkable results of pressure which the rocks have under- 

 gone. He shows how granitic veins » ere folded and crumpled, 

 how a kind of transver.-al stratification has arisen in beds of 

 stratified gneiss under the influence of pressure, and he con- 

 cludes, from an accurate study of the subject, that altogether the 

 rocks show a far greater degree of plasticity than might have 

 been supposed. "It seems that there ate masses, as, for in- 

 stance, the gneiss of Svenningdal, that have on one side a true 

 stratified structure (not merely parallel or schisto-e structure) 

 which could hardly be found in a rock of igneous origin, and on 

 the other side send veins, or have included fragments which 

 have undergone metamorphic changes." 



One of the most attractive features of M. Reusch's work 

 is the attention he has given to metamorphic phenomena 

 and to changes caused by the pressure undergone by strata 

 during their folding. The metamorphic phenomena were 

 especially studied in the O-oren district. The limestone 

 which contains Silurian fossils has become marble, and the cause 

 of metamorphism was not contact with some eruptive rock, tut 

 rather (as was observed in the Bernese Oberland by Swiss geo- 

 logists) pressure and the molecular movements which pressure 

 has occasioned in rocks. The clay, in which trilobites and 

 other animals were entombed at Vagtdalen, has become a rock 

 like muscovite schist with porphyritically inclosed clusters of mica. 

 As to the 'gneiss which appears among undoubtedly Silurian 

 rocks, the author is inclined to consider it as sedimentary and as 

 having been originally formed of loose materials. The granulite 

 is clearly stratified and of sedimentary origin. The changes 

 produced by the folding of strata and by the pressure they have 

 undergone, are described with much accuracy and illustrated by 

 many drawings. The fossils are nearly all compressed and 

 elongated ; the formerly conical coralla have received the shape 

 of flat elongated biscuits, in accordance with the direction of 

 pressure and stretching. The same is true with regard to all 

 other fossils. In the green conglomerate- at O-oren, all the 

 stones are flattened and elongated, acquiring thus a shape which 

 they could not have possessed originally ; very many of them 

 have such a shape as to give in a cross section the form of a 

 lance-shaped leaf. The same structure, remarks the author, mr.y 

 be observed with the aid of a micro-cope in the "hones" from 

 Eidsmarken in the South of Norway. Altogether the work cf 

 M. Reusch, although not rich in conclusions and generaluaticns, 

 will be a welcome addition to the accurate knowledge of the 

 still little understood metamorphic rocks. The Norwegian text 

 of this work is accompanied with a rather too short re'sume" in 

 English. P. K. 



CHEMICAL NOTES 



Carnelley [Chan. Soc. Jul. Trans., 1881, p. 317] has 

 repeated his experiments on the effect of pressure on the melting- 

 point of mercuric chloride, and has obtained results which show 

 that this salt cannot be obtained in the solid state at tempera- 

 tures appreciably above its melting-point. 



Jahn [Berliner Berichtc, xv. p. 1238] has made a series of 

 careful determinations of the den-ity of bromine vapour, which, 

 when compared with similar observations on chlorine made by 

 Ludwig, show that bromine vap nr does not attain the normal 

 density (Br = 79'95) until it is heated to 160° above its boiling- 

 point ; and also that although chlorine exhibits smaller diver- 

 gences from the normal density than bromine, it nevertheless 



