THE PORIF'ERA AND PROTOZOA 327 



glauconite. The formation of these goes on at the 

 present day on certain areas of the ocean floor. By 

 some means not yet fully explained this complex hydrated 

 silicate of iron, aluminium, and potassium is precipitated 

 in the interior of these minute tests, which themselves 

 may afterwards undergo solution. The resulting casts 

 form "green sands," or on a muddy bottom "green 

 muds." Similar deposits are found at many geological 

 horizons, but mosf abundantly in the Cambrian and 

 Cretaceous systems. 



As rock-forming organisms the Foraminifera are 

 important. A local example is the " spotted post " in the 

 Carboniferous Limestone of the north of England, full 

 of the arenaceous Saccammina cavteri. Of far greater 

 importance are the Fusulina-limestones of the Upper 

 Carboniferous, found in the Eastern Alps, Russia, China 

 and Japan, the East Indies, and the southern United 

 States. The arenaceous Orbitolina [Patellina] gives its 

 name to a limestone in the Aptian of Switzerland. The 

 White Chalk is made up in great measure of the 

 remains of Globigerina and Textulavia. The Eocene 

 Calcaire grossier of the Paris basin (the Paris building- 

 stone) is full of Miliolidae, and an Alveolina-limestone 

 occurs in the Hampshire Basin. Far more important 

 are the Eocene Nummulitic limestones, found throughout 

 the area of the great ocean (Tethys) of that period, i.e. 

 in the Atlas, Pyrenees, Alps, Carpathians, Balkans, 

 Egypt, Persia, the Himalayas, the East Indies and New 

 Caledonia. In many of these limestones Orthophragmina 

 is associated with Nummulites. The Oligocene Lepido- 



