242 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
traced by the boulder. The formation of such a crack would, of course, 
be facilitated by a pre-existing tendency of the rock to exfoliate follow- 
ing surfaces parallel to the crack, but such a coincidence would be very 
rare. The integrity of the rock-surface will henceforth be endangered 
because frost can work upon these cracks in the same manner as it 
works on joint-planes. The actual hollows would thus owe their existence 
to the postglacial splitting action of frost, prying up and breaking off 
prismatic fragments of the rock until these fragments had reached a 
thickness appropriate to the steep inner face of the lune. The size of 
the lune is limited by the distance measured along the crack through 
which the rock has been rendered unsound. 
The detailed features of the lunes seem to bear out this hypothesis. 
In every perfect or only partially completed furrow, a thin crack forms 
the prolongation of the gently sloping floor of the depression and is 
extended into the yet undisturbed rock at the horns where, in a little 
distance, the cracks disappear entirely. There is often a very decided 
difference in the freshness of the rock exposed on the steep and gentle 
slopes of the lune respectively. The latter may be distinctly weathered 
and lichen-covered, the former almost perfectly fresh and evidently so 
recently formed that no plant-life has had a chance to secure a foothold 
on the unaltered surface. This contrast between the two slopes is 
explicable by the hypothesis proposed 5 it forms a difficulty in the way 
of accepting any view which would derive a given lune in its present 
form from the direct action of the ice or its graving-tool. 
While it may be anticipated that most lunes will be developed in 
‘parallel orientation with grooves, it will, by the shear-hypothesis, be no 
less certainly expected that tensions will be set up by many boulders not 
travelling in the direction of general ice-movement, and that lunes with 
quite different orientations will result. As we have seen already, the 
facts agree with this expectation. A further cause for the typical devel- 
opment of furrows may be looked for in the structure of the rock on 
which they occur. It may be for this reason that the lunes of Hopedale 
are so well fashioned. There the glacial grooves cut across the schisto- 
sity uf the gneisses which will doubtless tear apart most easily in that 
transverse direction. Where the schistosity is not at right angles to 
the line joining lunes in sequence, the lunes are sometimes accordantly 
unsymmetrical, as if there were a tendency to exfoliation in a direction 
oblique to that line. Yet structure must play but a subordinate part in 
the manufacture of lunes, for they are found on such widely different 
rocks as the fine to medium-grained gneisses, coarse granitoid gneiss 
