442 



NATURE 



[October 6, 1910 



trasted the effect of low-level ice in deepening and mould- 

 ing valleys with the planing effect of high-level ice.' The 

 powerful influence of pre-glacial structures in determining 

 the course and character of ice-worn valleys was also 

 maintained by Dr. Becker, who attributed the Yosemite 

 and other valleys in the Sierra Nevada to the existence 

 of a vast system of joints, the decomposed rocks along 

 which have been removed by ice. Dr. Nordenskjold 

 insisted that long straight valleys like fjords can only be 

 <iue to ice erosion. 



The adjourned discussion, with Trof. Wahnschaffe in 

 the chair, was opened by Prof. .Salomon, of Heidelberg, 

 who remarked that erosion must take place where a 

 glacier presses firmly on the ground ; but we must wait 

 for the ice to withdraw before we can study its effects, 

 just as we have to wait for the dissection of a volcano 

 before we can see what has been going on in the depths. 

 He had seen cause fo change his mind, and to accept 

 the potency of glaciers as eroding agents, especially where 

 the rock-structures lend themselves to " plucking." 

 Joints in igneous rocks are not always evenly distributed, 

 and thus one part of the same mass may show erosion 

 while another resists. The suggestion of the action of 

 freezing water in the rock-joints under a glacier deserves 

 full consideration. Dr. von Dechy, from his studies in 

 the Caucasus, urged that much seeming erosion was due 

 to the clearing out of previously filled valley floors and of 

 lake basins by glaciers, and by catastrophic glacier-slides. 

 Prof. Wahnsc'hafl'e remarked that the Caucasian area was 

 not comparable with that north of the Alps, since no 

 great Piedmont glacier had formed north of the Caucasus. 

 Prof. Heim, in a vigorous speech, said the rock-surfaces 

 were palimpsests of river action and glacier action, and 

 the work of each was thus obscured. While stream 

 action concentrates itself in a portion of the valley floor, 

 a glacier spreads too widely to compare with it in erosive 

 power. So-called " plucked " masses had often merely 

 fallen from above on to the ice, and had come out below. 

 Glaciers have overriden Alpine landslides, but even then 

 without carrving many blocks away. The broad, rounded 

 form of glacier valley floors may even be due to_ the 

 wandering of a previous stream from side to side within 

 its valley walls. Then comes a glacier, and gives a final 

 touch to the form, overriding the taluses of a previous 

 age at either side. 



Prof. Hogbom, of Upsala, regarded the great chalk 

 masses, said to have been moved in northern Germany 

 and elsewhere by glacial erosion, as having been prepared 

 bv fractures. He compared a great glacier to the over- 

 thrust mass in mountain-building, and the ground moraine 

 to the breccia along the thrust plane. Erosion must ^ be 

 greatest under the vertical nose of an advancing glacier, 

 and not much under the glacier as a whole. Prof. E. 

 Stolley, of Brunswick, said the German chalk masses 

 represented genuine plucking and pushing forward. Lakes 

 due to glacial erosion occur even in the North Gerrnan 

 plain. Prof. Reusch confessed, like Salomon, to haying 

 changed his mind. He answered an objection in Helm's 

 speech by showing how a glacier must leave some up- 

 standing masses in its floor, and cannot be expected to 

 plane all equally away. Prof. Penck, on closing the dis- 

 cussion, accepted excavating action of subglacial streams, 

 especially along vallev sides, and urged that the only 

 differences between Prof. Heim and himself were now 

 really quantitative. 



T/ic Vrc-Camhrian Fauna. 



The discussion on the sudden appearance of the varied 

 Cambrian fauna shov/ed the firm belief in the evidence 

 of pre-Cambrian life as contended by Prof. Barrois from 

 the graphite of Brittany, by Dr. Sederholm from traces 

 of pre-Cambrian fossils in Finland, and by Prof. Roth- 

 pletz from the oolitic pebbles and organic traces in the 

 pre-Cambrian conglomerates of Sweden. The discussion 

 showed a general agreement as to the influence of the 

 absence of "carnivorous organisms from the pre-Cambrian 

 seas. Thus, according to Dr. J. W. Evans, creatures then 

 had no need of defensive structures, and according to Dr. 

 R. A. Daly there was, for the same reason, an accumu- 

 lation of decomposing organic matter in the early seas, .-ind 

 the resultant ammonium carbonate led to the precipitation 



of the pre-Cambrian limestones ; Profs. SoUas and Stein- 

 mann both thought that the early organisms had no hard 

 parts, which developed as the organisms became more 

 complex. Prof. Walther suggested that the pre-Cambrian 

 sea consisted of isolated basins, the waters of which 

 differed in chemical composition, and that organisms 

 living in water rich in silica secreted siliceous skeletons, 

 those in water rich in carbonate of lime formed calcareous 

 shells ; the phosphatic skeletons of trilobites and some 

 brachiopods were due to life in a sea rich in phosphate, 

 and chitinous shells were developed in fresh-water basins. 



In the section on general and regional geology. Dr. 

 Evans exhibited an elaborate and ingenious model to 

 illustrate the movements along the line of the San Andreas 

 fault during the recent Californian earthquake. It is 

 constructed of two sets of flexible wooden strips held 

 together by strings at their common edge ; the one part 

 was subjected to slow lateral stresses, and suddenly re- 

 leased from the strain by cutting the strings. Vibra- 

 tions were thus set up, the amplitudes of which were 

 greatest at the adjacent edges, and a musical note was 

 produced through the friction of metal attachments. Dr. 

 Evans believed the earthquake stresses to be of slow 

 accumulation, the larger vibratory movements after release 

 causing the sensible shocks, the frictional small tremors 

 the sounds. 



Prof. Hobbs gave a lecture on " The Fracture Systems 

 of the Earth's Crust," and urged their international 

 investigation, owing to their importance in relation to land 

 sculpture, the course of rivers, the discovery of obscure 

 faults, and earthquake disturbances. Prof. H. F. Reid 

 discussed the results of a recent paper on the California 

 earthquake, and exhibited a model similar in principle, 

 but not in construction, to that of Dr. Evans. Dissent 

 from his views as to the cause of the earthquake was 

 expressed bv Prof. Rothpletz, Dr. Oldham, and the chair- 

 man (Prof. 'Hobbs). 



An important paper by Prof. Tarr on the advance of 

 glaciers in Alaska as a result of earthquake shaking 

 indicated how the sudden advance and equally sudden 

 subsequent stagnation of many Alaskan glaciers might be 

 accounted for by masses of snow being shaken from the 

 n6\'& regions during the heavy earthquake of i8qq. In 

 discussing this paper. Prof. Freeh showed how the ex- 

 planation offered would account for the hitherto inexplic- 

 able sudden advances of the glaciers of the Alps. 



Dr. H. Stille described the earth movements in the 

 later rocks of north-western Germany, and showed the 

 influence of the Palaeozoic areas of the Rhine and the 

 Harz. 



Pre-Camhrian Geology. 



The petrographic section met on Saturday, in the morn- 

 ing under President van Hise, to consider the principles 

 of pre-Cambrian geology and the cause of regional meta- 

 morphism, and in the afternoon under M. Barrois, to 

 discuss pre-Cambrian stratigraphic classification. There 

 were fourteen papers and many speeches. The general 

 result of the morning's discussion was summarised at the 

 close by Prof. Cole as showing the great advance in recent 

 years of the views of Michel-Levy and Barrois as to the 

 formation of crvstalline schists by intense granitic injec- 

 tions, which in recent vears had been supported by Seder- 

 holm in Finland and his own work in Donegal. These 

 views were clearlv expressed bv a statement of the evidence 

 from Brittanv bv Prof. Barrois. Prof. Adams opened the 

 discussion by an account of the constant association with 

 the crystalline schists of vast granitic batholites, to which 

 he attributed the metamorphosis. Dr. Sederholm ex- 

 hibited a map of a Finnish islet on the scale of one- 

 twentieth of natural size, and he described the granitisa- 

 tion of the pre-Cambrian sediments by injection with 

 ''ranite when the adjacent rocks were half melted and 

 plastic. On the other hand, attention was directed to 

 intrusive gneisses elsewhere which had a less metamorphic 

 effect. Thus Prof. U. Grubenmann, of Zurich, contrasted 

 the actions of the gneisses of Scandinavia and Finland 

 with those of the .'Vlps, which had done less in melting 

 the adjacent rocks, but had a greater pneumatolitic effect. 



Prof Coleman described the alteration of conglomerates 

 at Sudbury, Ontario, into rocks that had been mapped as 



NO. 2136, VOL. 84] 



