686 THE POPULAR SCIENCE MONTHLY. 



era hemisphere) that "one of the "best criterions between the 

 effects produced by the passage of glaciers and of icebergs is boss 

 or dome-shaped rocks." 



3. Striated, grooved, and fluted rocks, though closely connected 

 with the preceding, form a distinct kind of evidence of the great- 

 est value. Most of the bosses of rock just described have been 

 exposed to the action of the atmosphere, perhaps since the ice left 

 them, and have thus become more or less roughened or even dis- 

 integrated ; but where the rocks have been protected by a cover- 

 ing of drift, or even of turf, and have been recently exposed, they 

 often exhibit numerous parallel striae, varying from the finest 

 scratches to deep furrows a foot or more in diameter. Fine ex- 

 amples are to be seen near the lakes of Llanberis, and they occur 

 more or less frequently in every glaciated country. Perhaps none 

 of the effects of ice so clearly demonstrate the action of glaciers 

 as opposed to that of icebergs, owing to the general constancy of 

 the direction of the striae, and the long distances they may be 

 traced up and down slopes, with a steadiness of motion and even- 

 ness of cutting power which no floating mass could possibly ex- 

 ert. Sir A. Geikie tells us that in Gareloch, Bute, and Cantyre 

 the striations on the rocks run up and over the ridges, and are as 

 clearly shown on the hill tops as in the valleys. Mr. D. Mackin- 

 tosh states (in his paper on the Ice-sheet of the Lake District and 

 of North Wales) that in the valley above Windermere the striae 

 cross Rydal Fell, Loughrigg Fell, and Orrest Head, ascending and 

 descending their slopes, often obliquely. But it is in the United 

 States that the most remarkable rock-groovings are to be found, 

 extending over a large portion of the northeastern States. In his 

 report on The Bock-scorings of the Great Ice Invasions Mr. T. 

 C. Chamberlin gives many fine illustrations, from photographs, 

 showing striae and grooves along sloping, curved, or vertical sur- 

 faces, the striae following the changes of curve, so that the grind- 

 ing material must have been slowly forced into close contact with 

 the irregular surface. Of one of these examples Mr. Chamberlin 

 says: 



The climax of adaptability is reached in the striation of warped and twisted 

 surfaces, and of tortuous valleys. One of the most remarkable known instances 

 of this within the limits of photographic illustration is furnished by the great 

 glacial grooves at Kelly's Island (Fig. 17). These exhibit not only the pliancy of 

 the ice, but at the same time its strong hold upon the armature with which it did 

 its work of abrasion, grooving, and striation. For, while these grooves can 

 scarcely be supposed to have been originated de now by the gouging action of the 

 ice, they are, nevertheless, plowed with deep furrows, the symmetry, continuity, 

 and peculiar form of some of which are only intelligible on the supposition that 

 they were cut by a single graving tool, held with sufficient tenacity by the ice to 

 execute by a single movement a deep, sharply defined groove. There is, perhaps, 

 no finer illustration of the pliancy with which the ice yielded to its encompassing 



