of the Ice of Glaciers, 243 



of the true glacier, modified according to its length and breadth." 

 Various explanations have been given of this characteristic 

 veined structure of glaciers. It has been supposed that the strata 

 formed horizontally by successive falls of snow and intervening 

 thaws in the neve or half- frozen snow out of which glaciers form 

 themselves, are by the unequal motions of the different depths of 

 the glacier tilted up into vertical positions and then over, so as 

 to incline forwards, and that the plane surfaces of these plates so 

 tilted are moulded by the pressure of the ice into the hollow 

 conoids of the veined structure. Mr. John Ball (than whom, in 

 Alpine questions, there is no greater authority) advocates this 

 theory, called that of stratification. 



The late Principal James Forbes attributes the veined struc- 

 ture to the same cause as the ripple in a stream of water, and 

 claims it in proof of the viscous theory of the descent of glaciers. 

 He formed " a viscous fluid of plaster of Paris mixed with glue 

 and ran it along an irregular channel or model valley which he had 

 constructed for the purpose. The relative velocities of the top and 

 bottom, and the sides and centre, of this pasty mass were shown 

 by alternating layers of two coloured pastes. He observed the 

 boundaries of the coloured pastes to be squeezed by their mutual 

 pressures into greatly elongated curves whose convexities were in 

 the direction of the motion • and in a medial section the retar- 

 dation of the bottom and the mutual actions of the posterior and 

 anterior parts shaped the bounding surface of the colours into a 

 spoon-like form"*. 



Dr. Whewell seems to have thought that the retardation of the 

 borders of a glacier would produce a tendency of the adjacent 

 molecules to move towards its centre, and that the curved form 

 of the veined surface is due to this tendency f. 



Professor Tyndall has propounded a theory of the veined struc- 

 ture which, to distinguish it from the viscous theory of Forbes, 

 he has named the " pressure theory." He has illustrated it as 

 follows : — 



" If a plastic substance, such as mud, flow down a sloping- 

 canal, the central portions will flow more quickly than the lateral 

 ones, which are held back by friction. Now the flow may be so 

 regulated that a circle stamped upon the central portion of such 

 a mud-stream shall move downwards without sensible distortion, 

 thus proving that the central mud is neither compressed nor 

 stretched longitudinally; for if the former, the circle would be 

 squeezed to an ellipse with its major axis transverse to the axis 

 of the stream ; and if the latter, it would be drawn out to an 

 ellipse with its major axis parallel to the line of flow. A similar 



* Occasional Papers, p. 78. 

 f Tyndall, < Glaciers of the Alps,' p. 396. 

 R2 



