DALY : GEOLOGY OF THE NORTHEAST COAST OF LABRADOR. 243 



and coarse granite of Rodney Mundy Island, the granitites of Sloop Har- 

 bor and Pomiadluk Point, the diorite of Aillik Bay, the gneiss and trap 

 of Hnpedale and the vein quartz as exposed near Ford Harbor. Each 

 rock has evidently acted as a more or less homogeneous body with 

 reference to the deforming force. 



Finally, it should be noted that postglacial frost is not of itself sufficient 

 to produce the systematic form and arrangement of the lunoid markings. 

 Examination of many ledges in Labrador on surfaces exposed by rifting 

 since the ice epoch, failed to disclose anything similar to the phenomena 

 described. To be sure, the familiar rough surfaces of ledges cleft by frost 

 acting on structural and less regular cracks, is sufficiently analogous to 

 the siu-face of a roche moutonnee covered with lunes, to suggest frost as 

 the common cause for the damage done, but the irregular depressions of 

 the first class lack the peculiar form and orientation of the second. 



This explanation of the lunes naturally suggested experiment, but 

 little has been done in that direction. When glass is scratched with a 

 diamond point, a multitude of curved transverse cracks is generally pro- 

 duced. They are always short, highly inclined, and recall in a significant 

 way the "serrated striae " of Andrews, 1 the "cross-fractures" of N. H. 

 Winchell, 2 the "crescentic cross-fractures" and "crescentic cracks" 

 described by Chamberlin. They may be allied to " chatter-marks " as 

 well. Russell has described others in the Sierra Nevada. 3 The convex- 

 ities of these curved cracks are always symmetrical to the glacial striye 

 and are directed upstream, just as the convexities of the cracks in the 

 glass of the experiment are symmetrical and directed in the sense 

 opposite to the movement of the diamond. The " crescentic cracks " 

 were seen in several localities in immediate association with lunes, but 

 with an invariably reversed attitude of the convexities. The former 

 class of markings, though commonly arranged in series, are much rarer 

 than the lunes, and seldom appear on any but the finer-grained and more 

 brittle rocks, quartzites, slates, and traps. They are usually under three 

 inches in length and thus much smaller than the lunes. Being simply 

 cracks, they do not form distinct hollows in the rock. Their steep 

 inclination doubtless explains the fact that the frost has not produced 

 such depressions as are likely to be developed where a crack has a low 

 dip into the rock-surface. 



While thus this experiment does not throw direct light on the inter- 

 pretation of the lunoid furrows, it yet suggests an explanation for a related 



1 E. Andrews, Amer. Jour. Sci. 1883, ser. 3, vol. 26, p. 101. 



2 Geol. Nat. Hist. Sur. Minn., 1884, vol. 1, p. 548. 



3 I. C.Russell, U. S. Geol., Sur. 8th Ann. Rept., 1889, p. 367. 



