68 



CLASSIFICATION. 



was necessary to comprise those beds which are 

 formed of the comminuted fragments of the primi- 

 tive rocks, but which contain few vegetable or ani- 

 mal relics. Such a class, therefore, w 7 as adopted, 

 under the name of intermediate or transition class. 

 Still later, owing to the progress of the science, a 

 third division was added, called the tertiary. So 

 that all the different rocks and strata that cover the 

 earth's surface are commonly arranged under the 

 following classes : 



1. Primary. 



2. Transition. 



3. Secondary. 



4. Tertiary. 



5. Basaltic and Volcanic. 



6. Diluvial and Alluvial. 



As, however, other classifications have been made 

 which are often referred to by geological writers, 

 it will be necessary to become acquainted with 

 them to a certain extent. Conybeare and Philips, 

 in their geology of England and Wales, divide the 

 crust of the earth into five portions, forming the 

 following classes : 



1. The superior, containing the tertiary deposites. 



2. The sup erme dial, containing the chalk, green 

 sand, Wealden, oolitic, and red sand formations. 



3. The medial, containing the coal measures, car- 

 boniferous limestone, millstone grit, and old red sand- 

 stone formations. 



4. The submedial, comprising the transition rocks. 



5. The inferior, containing the primitive rocks. 



M. de la Beche, one of the ablest geologists of 

 he present day, has made the following classifica- 

 tion of rocks, which has been adopted by many ge- 

 ologists as superior to any other. 



1. Stratified Rocks. 



£roupl. Alluvial. — Peat bogs, sands, coral islands 

 &c. 



