10 On Drift. 



or near the bottoms of some of the valleys, in which cases the 

 direction of the striae nearly coincides with those of the valleys. 



This coincidence of direction in the lower parts of the valleys 

 is exactly what we should expect, and is accordant with the cha- 

 racter of the like phenomena in Europe; and the persistency of 

 transverse oblique directions in the striae over the upper parts of 

 elevated tracts presents no difficulty ; for so long as the striating 

 agent (as an iceberg) should only come in contact with those upper 

 parts, its operations could not be influenced by the depths of the 

 valleys below. But what takes place at intermediate heights 

 between the bottoms of the valleys and the tops of the mountains % 

 It is impossible to suppose, if the side of a mountain were striated 

 in every part, that while the striae at the bottom should be parallel 

 to the lateral valley or axis of the mountain, and those at the top 

 should be, for instance, perpendicular to it, the strise at inter- 

 mediate heights should not have some intermediate directions in 

 passing from one extreme limit to the other. Careful observations 

 ought to be made on this point. The height to which the striae 

 preserve their parallelism with the valleys below, and the distance 

 from the tops of the higher ridges across which they preserve 

 their transverse directions should be most carefully noted. Nor 

 ought any geologist, in a delicate question of this kind, to trust 

 to vague measurements and general impressions. Every direction 

 ought to be carefully taken, and as carefully laid down on a good 

 physical map, together with the dip and strike of the striated 

 surface. The general configuration, too, of the immediate vicinity 

 should be described, with reference to its probable influence on 

 the motion of any mass to which the strise may be attributable. 

 Again, it has been said that in many cases the lee side and storm 

 side of an elevated ridge are sometimes equally marked by striae 

 transverse to its direction. This seems entirely at variance with 

 our observations on this side of the Atlantic, except in those cases 

 in which the striae are attributable to local action, in contradis- 

 tinction to that more general action of such agents as masses of 

 ice, for instance, driven in one direction over the whole region 

 from NW. to SE. I have not hitherto been able to represent 

 to myself the physical possibility of striae on the lee side remote 

 from the top of the ridge, having been produced by the general 

 action just referred to. May they not have been more frequently 

 due to local action than has been suspected ? The glacial theories, 

 on* their first introduction, did not, I think, make so much im- 

 pression on the minds of American as on those of European geo- 

 logists, and many of the recorded observations of striated rocks 

 were made, if I mistake not, under impressions very unfavourable 

 to those theories. Let me not be thought by this remark to cast 

 a reflection on American geologists — men to whom our science 

 owes so much, and from whom it expects so much more in the 



