178 FKAGMENTS OF SCIENCE. 



the cliff. This rested upon a bed of soft shale, which 

 extended round the base of the cataract. The violent 

 recoil of the water against this yielding substance 

 crumbles it away, undermining the ledge above, which, 

 unsupported, eventually breaks off, and produces the 

 observed recession. 



At the southern extremity of the Horseshoe is a 

 promontory, formed by the doubling back of the gorge 

 excavated by the cataract, and into which it plunges. 

 On the promontory stands a stone building, called the 

 Terrapin Tower, the door of which had been nailed up 

 because of the decay of the staircase within it. Through 

 the kindness of Mr. Townsend, the superintendent of 

 Groat Island, the door was opened for me. From this 

 tower, at all hours of the day, and at some hours of the 

 night, I watched and listened to the Horseshoe Fall. 

 The river here is evidently much deeper than the 

 American branch ; and instead of bursting into foam 

 where it quits the ledge, it bends solidly over, and falls 

 in a continuous layer of the most vivid green. The 

 tint is not uniform ; long stripes of deeper hue alter- 

 nating with bands of brighter colour. Close to the 

 ledge over which the water rolls, foam is generated, the 

 light falling upon which, and flashing back from it, is 

 sifted in its passage to and fro, and changed from white 

 to emerald-green. Heaps of superficial foam are also 

 formed at intervals along the ledge, and are imme- 

 diately drawn into long white striae. 1 Lower down, 

 the surface, shaken by the reaction from below, in- 

 cessantly rustles into whiteness. The descent finally 

 resolves itself into a rhythm, the water reaching the 

 bottom of the fall in periodic gushes. Nor is the 



1 The direction of the wind with reference to the course of a 

 ship may be inferred wi.th accuracy from the foam-streaks on the 

 surface of the sea. 



