GEOLOGY. 307 



Secondary Rocks. 



These are more numerous than the primary, but 

 twelve varieties include all that are usually found in 

 the British isles. They are always found recumbent 

 on the primary. 



1. Grawaucke. This consists of fragments of 

 quartz, or chlorite schistus, embedded in a cement 

 principally composed of felspar. 



2. Siliceous Sandstone. Consisting of fine quartz 

 sand, united by 'a siliceous cement. 



3. Limestone. This consists of carbonate of lime, 

 more compact in its texture than granular marble, 

 and c/ten abounding with marine exuviae. 



4. Aluminous Schist (or Shale). Consisting of 

 the decomposed materials of different rocks, cemented 

 by a small quantity of ferrugineous or siliceous mat- 

 ter, and often containing the impression of vegeta- 

 bles. 



5. Calcareous Sandstone. Calcareous sand ce- 

 mented by calcareous matter. 



6. Iron-stone. This is formed of nearly the same 

 materials as aluminous schist or shale, but containing 

 a much larger quantity of oxide of iron. 



7. Basalt. Consisting of felspar and hornblende, 

 with materials derived from the decomposition of 

 the primary rocks. The crystals are generally so 

 small as to give the rock a homogeneous appearance, 

 and it is often disposed in very regular columns, hav- 

 ing usually five or six sides. 



8. Bituminous or common coal. 



9. Gypsum or sulphate of lime, and often con- 

 taining sand. 



10. Rock-salt. 



11. Chalk, which usually abounds with the re- 

 mains of marine animals, and contains horizontal 

 layers of flints. 



12. Plum-pudding-stone. This consists of peb- 

 bles cemented by a ferrugineous or siliceous matter. 



