OUTLINES OF GEOLOGY. 



57 



gated by metallic oxides, and thus yields many of the most 

 valuable marbles. When argillaceous, it forms an excellent 

 hydraulic cement. Its organic remains are often petrifactions 

 of white carbonate of lime, which exhibit a beautiful appear- 

 ance when the mass is polished. 



136. The mineralogical character of the carboniferous 

 limestone is so similar to that of the limestone of the subme- 

 dial order, that in hand specimens they are frequently mis- 

 taken for each other. Hence, writers on geology have often 

 confounded them ; and the organic remains of the one have 

 been described as belonging to the other. Certain differ- 

 ences, however, are apparent, and others will probably be 

 detected. The family of belemnite, which prevails in the 

 formations of the supermedial order, is wholly wanting in the 

 carboniferous limestone. The family of ammonite is found 

 in the carboniferous limestone, but is wholly wanting in the 

 submedial order, and so we are inclined to believe, in oppo- 

 sition to high authority, are orthocerse, and the whole crus- 

 taceous family of trilobites. The most abundant family 

 among the fossils is the encrinite ; but this, except on a close 

 examination of the species, is far from being distinctive. 



137. The carboniferous limestone of Great Britain rises 

 to the height of mountains, and is sometimes named from 

 that circumstance. It is every where traversed by numer- 

 ous veins, and these are often rich in the metals. Lead, in 

 all its varieties, sulphuret and carbonate of copper, antimony* 

 zinc, iron pyrites, and hydrated oxide of iron are found in 

 this formation. These veins also contain many other valua- 

 ble or interesting minerals, as calc spar, arragonite, pearl 

 brown and fluor spars, selenite, carbonate and sulphate of 

 baryta, sulphate of strontia. Bitumen is also found, and it is 

 the only locality of the elastic variety of that substance. 



138. This formation occupies a vast extent in the United 

 States. Mr. Featherstonhaugh in his late tour, first struck 



