164 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
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 con- 
solidated 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 laminae 
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 genera] position of the experimental surface will be understood from the topo- 
graphical sketch (Plate VIII. fig. 1.) The theodolite was planted at a fixed point on the 
ice Q, just within the crevassed portion, which intervened between it and the western 
shore of the glacier. This point of fundamental and constant reference was fixed by 
an exactly vertical hole pierced with an iron jumper, or blasting iron, one inch in 
diameter, and was frequently deepened in order to preserve the centre as exactly as 
possible in the same vertical line in the ice. The theodolite was centred over it at 
every observation by means of a plummet, which nearly filled the cylindrical hole 
and permitted an adjustment, which one day with another might be accurate to 
about one-tenth of an inch. No stick was placed in the hole, but when not in use 
it was covered by a large flat stone, which effectually prevents congelation in ordi- 
nary weather-f~. The adjustment of the theodolite on the ice is always a matter for 
patience, but I succeeded in rendering it perfectly stable when once erected by in- 
serting the three feet in cavities in the ice, and filling them carefully with ice chips. 
* Third Letter on Glaciers, Edinburgh Philosophical Journal, October 1842, and Appendix to Travels, 1st 
edit. p. 407. 
t On one occasion this precaution having been neglected (in the case of a different mark on the ice), the 
hole was found completely frozen up after exposure to a day or two of severe weather in the month of August. 
It was however recovered by observing the beautiful stellar form of the ice-crystallization. 
