IN CLOUDS AND RIVERS, ICE AND GLACIERS. 



470. We have tracked the structure through 

 the various parts of the glaciers at which its 

 appearance was most distinct ; and we have 

 paid particular attention to the condition of 

 the ice at these places. The very fact of its 

 cutting the crevasses at right angles is sig- 

 nificant. We know the mechanical origin of 

 the crevasses ; that they are cracks formed 

 at right angles to lines of tension. But since 

 the crevasses are also perpendicular to the 

 planes of structure, these planes must be 

 parallel to the lines of tension. 



471. On the glaciers, however, tension 

 rarely occurs alone. At the sides of the gla- 

 cier, for example, where marginal crevasses 

 are formed, the tension is always accom- 

 panied by pressure ; the one force acting at 

 right angles to the other. Here, therefore, 

 the veined structure, which is parallel to the 

 lines of tension, is perpendicular to the lines of 

 pressure. 



472. That this is so will be evident to you 

 in a moment. Let the adjacent figure rep- 



FIG. 1!). 



resent the channel of the glacier moving in 

 the direction of the arrow. Suppose three 

 circles to be marked upon the ice. one at the 

 centre and the two others at the sides. In a 

 glacier of uniform inclination all these circles 

 would move downward, the central one only 

 remaining a circle. By the retardation of 

 the sides the marginal circles would be drawn 

 out to ovals. The two circles would be 

 elongated in one direction, and compressed in 

 another^ Across the long diameter, which 

 is the direction of strain, we have the mar- 

 ginal crevasses : across the short diameter 

 m n, which is the direction of pressure, we 

 have the marginal veined structure. 



473. This association of pressure and struc- 

 ture is invariable. At the bases of the cas- 

 cades, where the inclination of the bed of the 

 glacier suddenly changes, the pressure in 

 many cases suffices not only to close the cre- 

 vasses but to violently squeezs the ice. At 

 such places the structure always appears, 

 sweeping quite across the glacier. When 

 two branch glaciers unite, their mutual 

 thrusi i i tensities the pie-existing marginal 

 utructure of the branches, and develops new 

 planes of lamination. Under the medial mo- 

 raines, therefore, we have usually a good de- 

 velopment of the structure. It is finely dis- 

 played, for example, under the great medial 

 moraine of the glacier of the Aaf. 



474. Upon this glacier, indeed, the blue 

 veins were observed independently three 

 years after M. Guyot had first described 

 them. I say independently, because M. 

 Guyot's description, though written in 1838, 

 remained unprinted, and was unknown in 



1841 to the observers on the Aar. These 

 were M. Agassi z and Professor Forbes. To 

 the question of structure Professor Forbes 

 subsequently devoted much attention, and it 

 was mainly his observations and reasoning** 

 that gave it the important position now as- 

 signed to it in the phenomena of glaciers. 



475. Thus without quitting the glaciers 

 themselves, we establish the connection bo 

 tween pressure and structure. Is there any- 

 thing in our previous scientific experience 

 with which these facts may be connected ? 

 The new knowledge of nature must always 

 strike its roots into the old, and spring from 

 it as an organic growth. 



GG. SLATE CLEAVAGE AND GLACIER 

 LAMINATION. 



476. M. Guyot threw out an exceedingly- 

 sagacious hint, when he compared the 

 veined structure to the cleavage of slate 

 rocks. We must learn something of this 

 cleavage, for it really furnishes the key to 

 the problem which now occupies us. Let us 

 go then to the quarries of Bangor or Cum- 

 berland, and observe the quarrymen in their 

 sheds splitting the rocks. With a sharp 

 point struck skilfully into the edge of the 

 slate, they cause it to divide into thin plates, 

 fit for roofing or ciphering, as the case may 

 be. The surfaces along which the rock 

 cleaves are called its planes of cleavage. 



477. All through the quarry you notice the 

 direction of these planes to be perfectly con- 

 stant. How is this laminated structure to be 

 accounted for ? 



478. You might be disposed to consider 

 that cleavage is a case of stratification or 

 bedding ; for it is true that in various parts 

 of England there are rocks which can be 

 cloven into thin flags along the planes of 

 bedding. But when we examine these slate 

 rocks we verify the observation, first I be- 

 lieve made by the eminent and venerable 

 Professor Sedgwick, that the planes of bed- 

 ding usually run across the planes of cleav- 

 age^ 



479. We have here, as you observe, a case 

 exactly similar to that of glacier lamination, 

 which we were at first disposed to icgard as 

 due to stratification. We afterward, how- 

 ever, found planes of lamination crossing the 

 layers of the neve, exactly as the planes of 

 cleavage cross the beds of slate rocks. 



480. But the analogy extends further. 

 Slate cleavage continued to be a puzzio to 

 geologists lill the late Mr. Daniel Sharpe 

 made the discovery that shells and other fos- 

 sils and bodies found in slate rocks are inva- 

 riably flattened out in the planes of cleavage. 



481. Turn into any well-arranged museum 

 for example, into the School of Mines in 

 Jermyn Street, and observe the evidence 

 there collected. Look particularly to the 

 fossil trilobites taken from the s4ate rock. 

 They are in some cases squeezed to one third 

 of their primitive thickness. Nurner.ju~ 

 other specimens show in the most striking 

 manner the flattening out of shells. 



482. To the evidence adduced by Mr. 



