September i, 1898] 



NATURE 



415 



increased. And so the process continues from layer to { 

 layer up to the surface, the motion of which is not the i 

 sum of the differential movements of all the underlying 

 layers, but of part of the same. The surface as a 

 whole, therefore, has the greatest motion, although the 

 proper motion of the superficial stratum itself is the 

 least of all. 



Helmholtz would appear to have been the first physicist 

 abroad to recognise the significance of Thomson's theory, 

 and he set forth its application in such a form as could 

 not fail to attract attention. Since the publication of his 

 *■• Populare VVissenschaftliche Vortrage," however, so 

 much has been written on the subject of glacier motion 

 — so many conflicting explanations and criticisms have 

 appeared — that laymen may be excused if they confess 

 to a feeling of confusion in regard to the whole question. 

 We feel sure, therefore, that Dr. Drygalski's work will 

 be welcomed not by physicists only, but by all who 

 desire to have clear views on the subject with which it 

 deals. They will find in its pages excellent descriptions 

 and illustrations of the varied glacial phenomena, so that 

 even those who may not quite agree with some of his 

 conclusions will yet thank the author for the abundant 

 data he has supplied. 



To geologists, not the least interesting portions of Dr. 

 Drygalski's work are those that deal with glacial action. 

 He shows that the conclusion reached by them as to the 

 former existence of a great ice-sheet in Northern Europe 

 is justified, and that the conditions under which they 

 believe the "diluvium" was accumulated are reproduced 

 in Greenland at the present time. In Europe the ice- 

 sheet occupied the basin of the Baltic, its source being 

 in the lofty heights of Scandinavia to the north-west, 

 and Its termination in the regions lying south and east 

 — regions that slope up to heights of several hundred 

 metres and more above the bottom of the Baltic basin. 

 In Greenland the "inland ice" fills the depression be- 

 tween the mountains of the east and west coasts, the 

 former of which constitute a broad belt of high ground 

 that possibly e.xtends into the very heart of the country. 

 This mountain-tract is the source of the "inland ice," 

 the terminal front of the latter thinning off upon the 

 slopes of the less elevated mountains of the west coast. 

 The numerous deep fiords by which that coast is in- 

 dented, penetrate to the inland depression, and into these, 

 therefore, enormous ice-streams make their way. To 

 the great fiord-glaciers of Greenland there was nothing 

 analogous along the southern and eastern margins of the 

 old " inland ice " of Northern Europe. Between the 

 fiords of Greenland, however, the ice-sheet thins out 

 upon the mountain slopes in the same way as the 

 European mer de glace must have done upon the flanks 

 of the Riesengebirge and other ranges of Middle 

 Germany. 



The smoothed and striated surfaces observed under- 

 neath the edge of the " inland ice," and in the areas from 

 which it has retired, exactly recall those of Europe. 

 Their origin. Dr. Drygalski remarks, is not hard to under- 

 stand when we remember that the chief work of ice 

 movement is carried on at the bottom, where the relative j 

 motion is greatest. The bottom-layers of the ice are 

 crowded with rock-debris, which under glaciostatic pres- 

 sure is carried from areas where the ice is thickest 

 to regions where it is thinnest, and in this way it often 

 travels from lower to higher levels. Armed with this 

 material, the " inland ice " is a most effective agent of 

 erosion. As the included material increases in quantity, 

 the relative thickness of the ice is correspondingly 

 diminished, and thus changes in the direction of ice- 

 movement must take place. Hence erratics, after 

 travelling for some distance in some particular direction, 

 may change their course again and again. And so in 

 like manner divergent striit may be engraved upon the 

 rock-head over which the ice is moving. The varying 



NO. 1505, VOL. 58] 



configuration o< the land-surface is thus not the only 

 cause of changes in the direction of ice-flow. 



The author is convinced that "inland ice" is quite 

 capable of producing the contortion and disturbance 

 which so frequently characterise the diluvial deposits of 

 North Germany. Powerful pushing and shoving are 

 effected by the horizontal movement of the lowest layers 

 of an ice-sheet. Any water-saturated deposits under- 

 lying such a mass would be influenced in the same way 

 and subjected to the same disturbance as the debris- 

 laden portions of the ice itself. Where the ice is free 

 from inclusions the internal changes which result in 

 horizontal movement are not interfered with — the ice- 

 layers remain undisturbed. But when debris is present 

 the movements due to pressure are hindered and im- 

 peded, and the ice-layers amongst which it lies become 

 bent and folded. In alluvial or similar deposits under- 

 lying the ice folding would be still more readily pro- 

 duced, since in their case pressure is no longer relieved, as 

 in the ice, by transference of conditions, but is entirely 

 converted into mechanical deformation. 



The " inland ice " where it thms off upon the flanks 

 of the west coast mountains is bordered by moraines. 

 These are composed of materials derived from the bottom 

 of the ice-sheet, and are continually being added to ; the 

 moraines, in short, are gradually heaped up at and 

 underneath the thin edge of the ice-sheet. In other 

 places where the ice is bordered by precipitous land 

 no moraines are extruded, the steep rock-declivities 

 causing a deflection of the ice-flow. The moraines, 

 according to Drygalski, present the same appearances 

 as the " end-moraines " of North Germany. Although 

 for the most part unstratified, they yet now and again 

 consist in part of water-arranged materials. Scratched 

 and polished stones were common. It is clear, indeed, 

 from the author's descriptions that the rnorainic matter 

 extruded from the " inland ice" of Greenland has essen- 

 tially the same character as our boulder-clays. 



Dr. Drygalski draws attention to the interesting fact 

 that not only in the marginal tracts of the " inland ice," 

 but in certain independent glaciers the " blue bands," 

 which are the result of pressure, trend in the general 

 direction of ice-movement. This shows that there must 

 be pressure in the direction of the high grounds over- 

 looking the ice, and perpendicular to the trend of ice- 

 flow. The author thinks it probable, therefore, that 

 under these conditions subglacial morainic materials 

 might well be heaped up in banks and ridges having a 

 direction parallel to that of glacial movement. 



With regard to the ground-moraine itself, there can 

 be no question that this is partly carried in the lower 

 portions of the ice, and partly pushed forward under- 

 neath, and, further, that the forward movement must 

 result in the deformation of underlying unconsolidated 

 formations. The moving force is, of course, in the ice 

 itself. With the augmentation of included debris the 

 mobility of the mass is impaired, internal friction in- 

 creasing the more closely the materials are crowded 

 together. It is only when debris is well-saturated that 

 under pressure movements like those of the ice itself can 

 take place. In a compact subglacial mass of ddbris the 

 movement communicated by the flowing ice above must, 

 owing to friction, quickly die out downwards. Only a 

 relatively thin layer of ground-moraine, therefore, can 

 travel onwards underneath the ice. Immense quantities 

 of material, however, are interstratified with the lower 

 layers of the " inland ice," and these are eventually 

 added to the ground-moraine. The amount of this in- 

 cluded or intraglacial ddbris depends upon the thick- 

 ness of the ice, and must thus vary from place to place 

 As the ice diminishes in thickness, its ability to trans 

 port rock-materials declines, and the rubbish begins to 

 be deposited below. Dr. Drygalski thinks that the 

 boulder-clays of North Germany were in all probability 



