190 FRAGMENTS OF SCIENCE 



cataract was immediately observed. A thick layer of 

 limestone formed the upper portion of 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, under- 

 mining the ledge above, which, unsupported, eventually 

 breaks off, and produces the observed recession. 



At the southern extremity of the Horseshoe is a prom- 

 ontory, formed by the doubling back of the gorge exca- 

 vated 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 kind- 

 ness of Mr. Townsend, the superintendent of Goat 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 evi- 

 dently much deeper than the American branch; and in- 

 stead 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 alternating with bands of brighter color. 

 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 im- 

 mediately drawn into long white striae. ' Lower down, the 

 surface, shaken by the reaction from below, incessantly 



1 The direction of the wind, with reference to the course of a ship, may be 

 inferred with accuracy from the foam-streaks on the surface of the sea. 



