448 



SCIENCE. 



[N. S. Vol. V. No. 115. 



the shadow will be cast into the fog and appear 

 gigantic. This is probably an explanation of 

 the ' pseudo-aurora. ' 



H. A. Hazen. 

 January 29, 1897. 



[The above letter entirely mistakes the point 

 of Goode's explanation 'of the pseudo-aurora. 

 The fact that the electric lights have shields 

 above them, which cut off vertical rays, as 

 stated by Hazen, is irrelevant ; for Goode does 

 not think that the apparently vertical pseudo- 

 auroral rays are really vertical ; but that they 

 are due to oblique rays emitted from the light 

 at various angles of inclination, and reflected 

 from under surface of horizontal snow plates, so 

 that the locus of the reflection stands in a ver- 

 tical plane through the observer, and the light 

 wherever the observer is ; hence the subjective 

 impression that the ray is really a vertical beam 

 of light. There is no analogy between these 

 apparently vertical illuminated rays and the 

 true dark shadows mentioned by Hazen. — Ed. 

 Science.] 



greenland glacieks. 

 To THE Editor of Science: The angular 

 and apparently unglaciated peaks in Greenland 

 mentioned by Professor Tarr in your issue of 

 to-day are represented in Pennsylvania by simi- 

 larly angular ridges covered by angular and 

 local debris. It seems that advancing ice has 

 no power to surmount a moderately sharp 

 slope, but masses at its base and accumulates 

 till the summit is reached, when a thrust plane 

 is developed in the glacier above which the 

 moving mass proceeds across the summit. This 

 has been noted by the writer (Am. Jour. Sci., 

 March, 1895, p. 181) at Bethlehem and in Mif- 

 flin township. Since the publication of the 

 above other instances have been found which 

 show that the glacier pours into a valley and 

 fills it, or masses against a steep, opposing 

 slope, develops the shear and remains practi- 

 cally stagnant below the thrust plane, or would 

 remain so were it not for its ablation and the 

 erosion due to subglacial torrents, which cause 

 it to settle down the slope and down the valley 

 trough, and thus become an accentuated creep 

 which strews the valley with local fragments 

 from the summit. The constantly forming 



sub-glacial void, due to the causes just stated, 

 induces a downward movement in the ice 

 above the thrust plane, and the crest of the 

 ridge is frequently found crushed by vertical 

 forces. In the Mahanoy region the vertical 

 outcrop of hard sandstone is thus crushed flat 

 to a depth of ten feet on the crest, and bent to 

 north on the northern slope and to the south on 

 the opposite side. This is but one instance 

 where valleys have been glaciated while the 

 summits of the ridges remain angular, and the 

 fact that there is always difficulty in tracing 

 moraine lines over ridges may be accounted for 

 by the fact that ridge deposits are not allowed 

 to remain in situ but creep down the slopes to 

 the valley troughs. The finding of angular 

 ridges or peaks, therefore, is, as Professor Tarr 

 states, no sign of the absence of ice from the 

 locality. 



Edward H. Williams, jr. 

 Lehigh University. 



SCIENTIFIC LITERATURE. 

 V evolution de I' esclavage. Par Ch. Letour- 



neau. Paris, Vigot Freres. 1897. 1 vol. 



8vo. Pp. 538. 



It is a sad fact, emphasized by Professor Le- 

 tourneau, that in all times and places most of 

 the work of the world has been imposed upon 

 the minority of the inhabitants. In old times, 

 and in some places to-day, this was accom- 

 plished by the simple means of brute force, re- 

 ducing the conquered and the feeble to the 

 condition of slavery. The development of this 

 tendency in the jiast, and its possible future 

 effects, are the theme of the work before us. 



It begins with the lower species, pointing out 

 that in the societies of ants and termites there 

 are slaves and servile revolts, quite like those 

 in human history. Among men of the inferior 

 races — and not these only — the regular slave is 

 the woman. In many of the negi-o peoples she 

 is literally a beast of burden, and is rated no 

 higher than one. The women are bought and 

 sold; they are given away and, when incapable 

 of further profitable labor, are killed and eaten, 

 or turned out to starve. 



The long list of examples of this character 

 collected by our author leaves a disagreeable 

 sense of the meaness and baseness of masculine 



