THE BIRDS OF CLIPPERTON AND COCOS ISLANDS 503 



scattered about just above high-water mark have smoothed and 

 polished surfaces, though still preserving their branched shapes. 

 Lower down the coral fragments are worn by the waves to 

 rounded pebbles, and finally ground up into sand. All around 

 the island a sloping reef runs outward from high-water mark. 

 Its variable width is generally equal to or slightly greater than 

 the part of the island above water. Soundings at the anchorage 

 on the north side of the island at a distance of seven hundred 

 feet from the outer edge of the reef show depths of 38, 46 and 

 60 fathoms, while at the edge of the reef, just outside of low- 

 water mark, the depth is only about 20 fathoms. 



Near the middle of the east side of the island is a large mass of 

 dark igneous rock, about sixty feet high, which is not found else- 

 where on the island. It is much eroded by water ; its exterior is 

 cut into numerous pinnacles and irregular projections, and its 

 interior is hollowed out by caves. Large passageways extend 

 entirely through the mass. The rock is so worn by the water 

 that it has the appearance of being a large isolated block of 

 sandstone. It is, however, simply the surviving remnant, 

 above water, of the original volcanic island on which the coral 

 atoll has been built. In places the surface of the rock is decom- 

 posed into a soft, whitish material, easily broken between the 

 fingers. The excrement of birds upon it in other places has 

 made on the surface a white, glassy formation. 



Boobies and terns inhabit the island in enormous numbers. 

 The phosphates of their excrement, for they feed almost entirely 

 on fish, have acted chemically on the calcium carbonate form- 

 ing the coral surface of the island, and have in places trans- 

 formed large beds of this coral into calcium phosphate. The 

 island was leased from Mexico in 1898 by the Pacific Islands 

 Company for the purpose of exporting this coral phosphate for 

 fertilizing purposes. Where good formations are found the 

 mixture is dug up, broken into small pieces, dried, sacked and 

 shipped without further preparation. The chemist on the island, 

 Mr. F. T. Shepherd, told us that this exported product yields 

 from 70 to 80 percent of phosphates. 



We visited Clipperton Island November 23 and 24, 1898, 

 and to the representatives of the Pacific Islands Company sta- 



