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 ofteu a very decided 

 difference in the freshness of the rock exposed on the steep aud 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 ; 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 of 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 



