60 



OUTLINES OF GEOLOGY. 



we have ranked with the former, might with equal conve. 

 nience be placed here. Omitting it, this order may be di- 

 vided into two groups, transition limestone and slate. 



145. The transition limestone resembles in chemical and 

 external characters, the carboniferous. So far as we have 

 been able to judge, it is more frequently crystalline in its 

 structure. It is generally of a dark gray colour, and some- 

 times quite black. This colour arises from carbon and bitu- 

 men disseminated throughout the mass. The pure limestone 

 often passes into one containing carbonate of magnesia, or 

 much silicate of alumina. 



146. The transition limestone of the United States con- 

 tains numerous nodules of hornstone. It does not appear to 

 be rich in metals, although some copper mines have recently 

 been found in this formation in Virginia. Brogniart calls it 

 rich in the useful metals, but he appears to confound it with 

 the carboniferous limestone. 



147. The organic remains of this group are very numer- 

 ous. Of vertebrated animals, no others occur except a few 

 fish. ThemolluscaB are principally, if not wholly, chambered 

 on multilocular. Of these, the ammonite is wholly wanting, 

 and is replaced by the orthocera. Of chambered bivalves the 

 productus is the most remarkable, but is said to be also found 

 in the carboniferous groups. Among articulated animals is 

 found the family of trilobites, which was probably crusta- 

 ceous, and of these the calymene of Blumenbach is decided- 

 ly characteristic. The discovery of this by myself at Tren- 

 ton Falls, identified the great limestone formation of the Apa- 

 lachian group of mountains with that of Dudley, and, in conse- 

 quence, with the transition rocks of Europe. From the same 

 place was brought a new and remarkable trilobite, which 

 was described by Dr. Dekay under the name of isotelus. 



Of radiated animals, the family of encrinites is extremely 

 abundant, and the rocks of Trenton Falls containing coralli- 



