MICROSCOPIC OBJECTS. 95 



Fig. 189. Section of Sandstone from Cherbourg Works, 

 France, x 40. 



The drawing is taken from the section of this rock as seen 

 under polarized light. This rock is a true siliceous sand- 

 stone, and is composed of subangular fragments of nearly pure 

 quartz, cemented together by a small amount of siliceous 

 matter ; it also contains occasional specks of clay and oxide 

 of iron. 



The microscopical investigation of rocks is of great value 

 to the geological student. 



Fig. 190. Siliceous Infusorial Earth from Bilin, Bohemia, 



x400. 



There are found in many parts of the world rocks composed 

 almost entirely of minute siliceous skeletons of an order of 

 confervoid Algae called Diatoms (compare the drawing with a 

 recent form of Diatom represented at fig. 98) . These minute 

 organisms were long thought to belong to the animal kingdom; 

 and it is only within the last few years that they have been 

 thoroughly studied and classified. 



This rock, like chalk, belongs to the sedimentary formations. 



