850 THE JOURNAL OF GEOLOGY. 



the drift, their absence must be due to one of two things ; either 

 they were never formed, or they have been destroyed. Where 

 the character of the underlying formation is such as to unfit it 

 for receiving striae, they could never have been developed. This 

 would be true where the underlying formation is loose sand or 

 gravel. Where the underlying rock is such as not to favor the 

 development of striae, but not such as to prevent their forma- 

 tion altogether, they may be rare. This might be true of rock 

 which is unusually hard, and for this reason difficult of stria- 

 tion, or it might be true of rock which is very fragile and easily 

 crushed. Where the underlying rock is ill-adapted for retaining 

 striae, they could not be expected to remain in great numbers, 

 even if once developed. This would be the case where the rock 

 is plastic, like clay, or where the rock is readily disintegrated. 

 Again, striae which once existed on rock adapted to receiving and 

 retaining them may have been destroyed because of adverse con- 

 ditions. Thus limestone received and has preserved striae quite 

 as well as any formation where the striated surface has remained 

 deeply covered with drift. Where little or no covering has pro- 

 tected it, the striae are absent, or ill-defined. It is reasonable to 

 suppose that such surfaces were once striated, but that weather- 

 ing has destroyed the marks. 



The foregoing considerations help to explain why the striae 

 are not found at all points beneath the drift, even if the drift 

 agencies and the polishing and striating agencies were the same, 

 as we must believe they were. But there are places, and not a 

 few of them, where striae and polishing do not appear to have 

 existed, even though the bed rock is of such a nature as to have 

 favored their development and retention. In such cases the 

 drift is often seen to rest, not on the solid rock, but on a layer 

 of residuary material which originated in the decomposition of 

 rock beneath. In this case the immediate substratum of the drift 

 is really an incoherent earth, incapable of receiving striae. In 

 other areas where striae and polishing are not known, their 

 absence is probably apparent only. This is true in regions 

 where the rock is so deeply buried that it is rarely seen. 



