670 



The author divides the questions of structure into three principal 

 cases : 



1 st. Marginal Structure, produced by pressure due to the swifter 

 motion of the centre of the glacier. 



2nd. Longitudinal Structure, produced by pressure consequent 

 on the mutual thrust of two tributary glaciers ; developing veins 

 which run parallel to the direction of the trunk stream. 



3rd. Transverse Structure, produced by pressure due to the 

 change of inclination, and the thrust from behind, endured by 

 glaciers at the bases of the ice-falls. 



The author also gives a physical analysis of the mode in which 

 the pressure produces the structure. He shows experimentally that 

 planes of liquefaction are produced in ice at right angles to the direc- 

 tion of a pressure acting upon the mass. In the glacier these planes 

 of liquefaction are the channels by which the air is ejected, and the 

 blue veins produced. 



A section of the Paper is devoted to the consideration of the shape 

 of the bubbles entangled in glacier ice ; as affording evidence of 

 pressure. The author has endeavoured to refer the observed facts 

 to their true cause, and to show that the conclusions hitherto drawn 

 from this remarkable phenomenon are untenable. The shape of the 

 bubbles furnishes no ground for any conclusion regarding the 

 pressure to which the mass containing them has been subjected. 



The Paper also includes a short section containing remarks on 

 glacier motion ; in which it is shown that this motion is of a com- 

 posite character ; being partly due to the sliding of the glacier over 

 its bed, and partly to the yielding of the ice under severe pressure. 

 A brief section is also devoted to the explanation of the Dirt-bands 

 of the Mer-de-glace. 



