ARIZONA DIATOMITE. 
BY WILLIAM P. BLAKE, E.G.S., 
Professor of Geology, University of Arizona. 
In a recent paper 1 descriptive of the occurrence, composition 
and uses of the extensive beds of diatom-earth in the valley 
of the San Pedro , 2 Arizona, the paleontology of the deposits 
was necessarily omitted. In this communication I attempt to 
supply that deficiency and to illustrate some of the more im¬ 
portant and novel forms by micro-photography. 
The fossils occur in thick beds many square miles in area, 
in horizontal layers cut through by ravines, and probably one 
hundred feet in thickness. These beds when freshly broken 
are snow-white and chalk-like in appearance, but are siliceous 
and not calcareous in composition. Under the microscope the 
diatoms are seen to be distributed through, or mingled with, 
nearly colorless vitreous particles, apparently a very finely di¬ 
vided volcanic ash or dust such as may have been wafted by 
the wind and deposited in a lake or estuary of quiet water. 
The siliceous shields or frustules are easily separated from this 
vitreous magma and form beautifully perfect objects in the field 
of a microscope of high power. 
Full suites of samples were submitted to eminent specialists, 
who have expressed great interest in this discovery, and have 
1 Diatom Earth in Arizona. W. P. Blake. Trans. Am. Inst. Min. 
Engrs. 1902. 
2 The Rio San Pedro of the early explorers and fathers is the Rio 
Quiburi of the aborigines. The valley was explored in 1697 by two 
Spanish parties, united for the purpose, and accompanied by 30 Indian 
auxiliaries. They marched down the river to the Gila, and thence to 
Casa Grande, returning up the Santa Cruz. (See Bancroft's History, 
vol. xvii., p. 355.) 
