Burbank.] 154 [Nov. 19, 



If we accept the glacial theory of the drift, we have to consider 

 the effects that would be produced by a great ice sheet passing over a 

 surface like that here described. It is evident that if in the first part 

 of its course the mass of ice cut its way down to the solid ledges, 

 sweeping the decomposed material before it, the vast amount of this 

 material would soon form a mountain ridge that must impede its pro- 

 gress; and the ice being the lighter material would be lifted from the 

 surface and pass over and leave beneath it a great portion of the de- 

 composed mass. 



If, according to the most probable theory, the glacial climate came 

 on gradually, and in each successive year the ice sheet extended far- 

 ther southward, the annual advance and retreat would still fail to 

 sweep the ledges clear of decomposed materials, except in the high- 

 est and most exposed situations. Yalleys and ravines, especially in 

 protected situations on the southern slopes of hills, instead of being 

 excavated to a greater depth, would become filled with the materials 

 of the terminal moraine, and these might remain undisturbed by the 

 ice subsequently passing over them. The great bulk of the material 

 would not be carried forward and deposited near the southern limit 

 of the glacier's extent, but would remain beneath the ice, to be gath- 

 ered and thrown into ridges during the retreat of the ice sheet as a 

 whole, at the close of the glacial period. 



The boulders of decomposition that have been referred to, the 

 fragments of quartz from veins, and all undecomposed masses of rock 

 in the material swept forward, would of course be more or less worn 

 and changed in form; but the rounded forms of many of the larger 

 boulders of the drift may, as we have seen, be accounted for in an- 

 other way. The concretionary structure which has been referred to 

 as determining the form and size of boulders, is, I believe, more com- 

 mon in the crystalline rocks than has been generally supposed, 

 though the frequent occurrence of this structure is a fact recognized 

 by the best authorities in geology. 1 



Boulders of decomposition produced by weathering, which takes 

 place at first along the joints, have been well described by Professor 

 Hartt, and no doubt the rounded forms are, in many cases, produced 

 in this way, independently of any thing like a concretionary structure. 

 This is especially the case in rocks abounding in compounds of the 

 protoxide of iron, which by their tendency to chemical change, aid in 



1 See Dana's Manual of Geology, p. 98. 



