450 



PRINCIPLES OF STRATIGRAPHY 



practical exclusion of others. Such are tlie great beds of Pen- 

 tameriis ohlonyus extending widely through the Siluric of Gotland 

 and New York ; the beds of Anoplotheca in the Clinton of the 

 Rochester region, and those composed of Exogyra or other shells 

 in the Cretacic of Texas ; the limestones entirely made up of Tur- 

 ritella mortoni in the Eocenic of Virginia, and numerous other ex- 

 amples. Rocks of this type generally form thin beds among the 

 clastic sediments. From the nature of the material, these beds can 

 be only rudely stratified, and irregularities in thickness may be ex- 

 pected where wave action has not been strong. 



Calcareous and Siliceous Oozes. These often form beds of 

 considerable thickness, as in the case of the chalk. These accumu- 

 lations when pure always indicate either a remoteness of the shore, 

 with perhaps deep-water conditions, or a very low relief of the land, 

 which has been peneplaned to such an extent that no clastic sedi- 

 ments are derived from it. The oozes formed in the modern ocean 

 include the following types : 



1. Calcareous: a, Foraminiferal; b, Pteropod ; c, Entomostra- 

 can ; d, Coccolith, and e, Rhabdolith ooze. 



2. Siliceous: a, Radiolarian ; b, Diatom. 



I. The Calcareous Oozes. 



Recent Foraminiferal Oozes. These are represented in the mod- 

 ern sea by the Globigerina ooze (Fig. 105), so named from the pre- 

 ponderance of the shells of Globigerina, which is represented by ten 



Fig. ioi. Globigerina bulloides. x 40. 

 (After Wyville Thomson.) 



Fig. 102. Orbulina universa. x 40. 

 (After Wyville Thomson.) 



species, while Pulvinulina is represented by five species. The total 

 number of species of foraminifera making up this ooze is only 21, 

 distributed in 9 genera, but only four species are of great impor- 

 tance in the making of this ooze. These are Globigerina bulloides 



