1845.] RIBBONED STRUCTURE PRODUCED IN SITU. 105 



invariably tends to disappear when a glacier becomes so crevassed 

 as to lose horizontal cohesion, as when it is divided into pyra- 

 midal masses. Now this immediately follows from our theory ; 

 for as soon as lateral cohesion is destroyed, any determinate 

 inequality of motion ceases, each mass moves singly, and the 

 structure disappears very gradually."* Now the ice at the 

 point in question is the compacted ice which has just passed 

 round the great promontory of Trelaporte, having been rent 

 by numberless chasms, and which is consolidated by pressure 

 in the bay in question, whilst the centre of the glacier being 

 still on the steep is deeply crevassed. The structure of the 

 even ice is continuously striped .with a regularity comparable to 

 that of the finest chalcedony for a distance of some hundred 

 feet. This structure must have been produced on the spot, since 

 no such perfect structure exists higher up, and if it did, [it] must 

 have retained all the marks of dislocation due to the formation 

 and reconsolidation of the fissures, which are so numerous and 

 wide as to render the passage of the glacier quite impracticable 

 if we follow the same strip of ice up towards the promontory of 

 Trelaporte. Let it then be recollected that the structure is 

 produced here, under our eyes, on the very spot where the 

 experiments about to be detailed were made, and that the 

 structure in question produced a vertical slaty cleavage so 

 distinct, that the ice broken into hand specimens may be split 

 parallel to it like any slaty rock, and that the fine hard lamina 

 projecting vertically after the glacier has been washed by rain, 

 permitted the blade of a knife to be thrust between them to a 

 depth of several inches, although they are rarely more than a 

 quarter of an inch thick. 



I shall now describe the actual measurements made upon 

 the glacier, in order that my method of proceeding in similar 

 cases (when I have only published results) may be understood. 



The general position of the experimental surface will be 

 understood from the topographical sketch (Plate III. fig. 1). 



* Third Letter on Glaciers, Edinburgh Philosophical Journal, October 1842, 

 [page 24 of the present volume]. 



