52 



AQUEOUS AGENCIES. 



surrounding ice, because more absorbent of beat, and tbus form deep 

 holes. 



An admirable illustration of extreme inequality of the surface of 

 ice is seen in the case of the small residual glacier still remaining on 



r"3s^> — 



Fig. 40.— Ice-Pillars on Parker Creek Glacier, California (after Russell). 



Mount Lyell, Sierra Nevada.* On the top of Mount Lyell there is an 

 immense amphitheatre (cirque), filled with snow and ice. In August 

 the surface of this ice-field is set with ice-blades, three to four feet high 

 and only two feet apart, as in the section Fig. 41. They are probably 



formed as follows : In winter, when the 

 snow is deep and light, it is blown into 

 wind-ripples on a large scale. These soon 

 become fixed by surface melting and freez- 

 ing, and then the greater action of the 

 sun in the troughs, partly by the reverber- 

 ation of heat and partly by accumulation 

 of dust there, causes these to become 

 deeper and deeper. It is necessary to re- 

 member that there is little snow or rain in this region after about the 

 first of May until November. 



Again, fissures or crevasses, often of great size, ten to twenty feet 

 wide, one hundred feet deep, and sometimes running entirely across the 

 glacier, are very abundant. As the surface of the glacier is often cov- 

 ered with snow, and the fissures thus concealed, they form the most 

 dangerous feature connected with Alpine travel. The law which gov- 

 erns their formation will be discussed hereafter ; suffice it to say that 



Fig. 41. 



* See paper by the writer, American Journal of Science, vol. v, p. 333, 1873. 



