66 SUBCARBONIFEROUS SYSTEM. 



almost wholly a mechanical deposit of very limited distribution and enormous thick- 

 ness. There are ripple-marks and other evidences of shallow water in different 

 strata. The fossils characteristic of it are Aneimites obtu&us, Amnigenia cofitxIcUlerm*, 

 Holopti/rhliis wmeriettmu, H. tayhri, and Diptemt xherwoodi. 



S 137. The total maximum thickness of the several Groups belonging to the 

 Devonian as given above is 14,500 feet, though no single section would furnish 

 such a depth. The greatest thickness is in Pennsylvania, and next in New York. 

 The thickness at Gaspe, Canada, is 7,036 feet, and the divisions into Groups are 

 not well defined. In the Western States several Groups are missing, and the thick- 

 ness of the rest is only a few hundred feet. All the strata are marine ; no land 

 or fresh-water shells have been found within them, and the land-plants are fairly 

 supposed to have drifted to the places where they occur. The Devonian is every- 

 where unconformable with the superimposed Subcarboniferous, which always begin 

 with a conglomerate or sandstone. The great reef-forming Corals so conspicuous in 

 the Upper Helderberg and Hamilton, did not survive the era. Cystideans became ex- 

 tinct. The family Sjririferidie, which commenced in the Upper Silurian, became 

 most prosperous in this age, and lived until the Jurassic. The three most notable 

 steps in the progress of development are found in the growth and abundance of 

 land-plants, the appearance of insects, and in the introduction and diversity of 

 fish. The Devonian fish belong to the Selachians or cartilaginous fishes, the 

 Ganoids, or fishes covered with plates or bony scales, and the Placoderms. 

 There is nothing known in connection with plants or animals indicating the tem- 

 perature of the sea, or climate on land, was different then from what it is now. 



CHAPTER XXV. 



SUBCARBONIFEROUS SYSTEM. 



S 138. This System was named and defined by David Dale Owen in 1838, 

 in the Geological Survey of Indiana. He found it to consist of massive sandstones, 

 limestones, and shales, lying between the Devonian and the Coal Measures, to be 

 characterized by Pentremites and other peculiar fossils, and to be capable of sub- 

 division into Groups. The name Subcarboniferous indicates its position is below 

 the Coal Measures. In the great valley of the Mississippi it is divided, in ascending 

 order, into Waverly, Burlington, Keokuk, Warsaw, St. Louis, and Kaskaskia 

 Groups. These Groups have been fully defined in Illinois, Iowa, Missouri, Ar- 

 kansas, Indiana, Ohio, Kentucky, and Tennessee, andean be determined with more 

 or less satisfaction beneath the Coal Measures in the four larger coal-basins, though 

 not throughout their whgle extent. For example, while the Groups are not dis- 

 tinctly marked in Pennsylvania, they can be readily determined on the oppo- 

 site side of the basin in Kentucky and Tennessee. This is because the rocks con- 

 sist largely of sandstones and shales in the east, which did not preserve well the 

 fossils, while in the west they are principally limestones, containing fossils in great 

 profusion and perfection. In Pennsylvania the sandstones and shales have a thick- 

 ness of 5,000 feet, which thin westerly and southerly, and gradually give way to 

 limestones and deep marine deposits. 



