DIATOIM AND PHOSPHATE DEPOSITS 461 



or about 6.4 per cent, of the entire sea floor, are covered with this 

 deposit. 



Diatoms Hkewise Hve in fresh water and deposits of diatomace- 

 ous ooze may form under such conditions. 



Fossil Diatomaceous Earths of Both Fresh and Salt Water 

 Origin. These are widely distributed over the world, especially in 

 Tertiary deposits. They ordinarily go under the name of Infusorial 

 earth or Tripolite, so named from the deposits at Tripoli in North 

 Africa. The most extensive is that found at Richmond, Virginia, 

 which extends for many miles, and is in some places at least 40 feet 

 in thickness. Other deposits are found in the Great Basin in Ne- 

 vada, Oregon, and California, where they form beds of great thick- 

 ness usually interstratified with volcanic material. 



Phosphate Deposits. 



Guano. A subordinate type of zoogenetic rock is the guano, 

 formed from the droppings of innumerable birds which inhabit iso- 

 lated islands and promontories on the sea coast. Near Iquique, 

 Province of Tarapaca, Chile, such a deposit with a thickness of 10 

 meters is known, and it is estimated that it may have formed in 

 1,100 years. The deeper strata of these formations are generally 

 darker in color and more crystalline. Deposits of the excrements of 

 seals sometimes alternate with those of birds in certain places on the 

 coast of Tarapaca. Here these deposits contain numerous smooth 

 fragments of porphyry from 3 to 10 cm. in length, derived from the 

 stomach content of these seals. (Tschudi in Walther-90 :6'5j.) 



A number of distinct phosphate minerals are found in guano. 



BIBLIOGRAPHY X. 



(See also Bibliography XI.) 



1. AGASSIZ, ALEXANDER. 1883. The Tortugas and Florida Reefs. 



American Academy of Arts and Sciences, Memoirs, Vol. XI, pp. 107-134 

 (Abstract, American Naturalist, Vol. XVII, pp. 1267-1268, 1883). 



2. AGASSIZ, A. 1888. Three Cruises of the Blake. Vol. I. Bulletin of the 



Museum of Comparative Zoology, Harvard College, Vol. XIV. 



3. AGASSIZ, A. 1889. The Coral Reefs of the Hawaiian Islands. Bulletin 



of the Museum of Comparative Zoology, Cambridge, Vol. XVII, pp. 

 121-170. 



4. AGASSIZ, A. 1893. A Reconnaissance of the Bahamas and of the Elevated 



Reefs of Cuba, etc. Bulletin of the Museum of Comparative Zoology, 

 Vol. XXVI, pp. 1-203, with plates and maps. 



