61 



it includes in some localities layers of an ashy tinge, which are, 

 however, not inferior to the rest of the deposit in the abundance 

 of their minute organic forms. It has throughout a tendency to 

 lamination in a horizontal direction, and toward its upper limit 

 this structure is so distinct as to cause it readily to separate in 

 thin crumbly plates. But of all its mechanical peculiarities its 

 great lightness is the most characteristic. From experiments 

 made many years ago, Prof. Rogers found that when pure and 

 quite free from moisture this material in its ordinary state of 

 compactness has a weight only one third as great as an equal 

 bulk of water. The minute siliceous fossils for which this deposit 

 has long been noted, belong, as is well kno^vn, almost entirely 

 to the family of Diatomacea3, and include a very large proportion 

 of Coscinodiscus and allied forms, whose exquisitely thin plates 

 lying in parallel positions in the mass have probably contributed 

 to the laminated structure before referred to. The number of 

 such frustules and other siliceous skeletons in each cubic inch of 

 the pure material can only be reckoned in millions, and a cubic 

 foot would contain a multitude far exceeding in number the en- 

 tire human population of the globe. 



The following description of the series of strata as exposed in 

 the principal ravine before referred to, will serve to illustrate the 

 relation of the Infusorial deposit to the others with w^hich it is 

 associated, and will at the same time illustrate the nature and fossil 

 contents of these Eocene and Meiocene strata. 



1. The lowest bed, which is seen resting directly on a soft 

 sandstone and conglomerate, consists of a mixture of sand and clay 

 having a yellowish gray color occasionally mottled with brown, 

 and including little irregular patches of green sand. This stra- 

 tum is crowded with the impressions of Turritella Mortoni, Car- 

 dita planicosta, and other well-known Eocene forms, which are, 

 however, so fragile as scarcely to admit of being preserved. 

 The thickness of this bed varies from six to ten feet. 



2. Next above we iind a stratum of dark olive or greenish 

 clay mixed with siliceous sand and containing diffused granules 

 and smaller particles of green sand. This bed abounds in the 

 teeth of squaloid fishes, especially of the genus Otodus and Odon- 

 taspis, and with Coprolites, mostly small and apparently derived 



