434 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



The upper members of this group consist of an argillaceous marl 

 of a light grey colour, and the lower of a greenish sandy marl, very 

 dark-coloured when wet, but of a light green-tinted grey when dry. 

 Its thickness has not yet been ascertained, but Dr. Troost, in his Third 

 Report to the Tennessee Legislature, states that it has been exca- 

 vated in search of water to a depth of eighty feet without penetra- 

 ting through it. In no instance, I believe, has either the greensand 

 or marl been discovered east of the Tennessee river, but it exists, ac- 

 cording to Troost, under the superficial soil, in most of the counties 

 west of that river, extending probably west and south into the states 

 of Mississippi and Alabama. On the west side of the Mississippi 

 river this formation sweeps off in a west course across the northern 

 tributaries of the Arkansas river*. 



Both the marl and greensand are rich in fossils. The most cha- 

 racteristic and most common shells of the marl are species of Exo- 

 gyra. One of these is a large species, E. costata of Say, and there is 

 another from near Washington in Arkansas, a different species, but 

 doubtless from the same formation. Other fossils giving character 

 to this formation are, Ostrea vesicularis, Brongt., Ostrea sandalina, 

 Goldf., Ostrea resembling O.falcata of Morton, Gryphcea convexa. 

 Say. Exogyra costata, Ostrea sandalina and Gryphcea convexa are 

 common to the marl and greensand. Besides these, there occur in the 

 latter stratum, Ostrea acutirostris, Nils., Ostrea Jlabellula, Lam., Ostrea 

 lateralis, Nils., O^^rea larva, Lam.; 2.Crassatella\ 2iNatica, species un- 

 determined; Delphinulal \ ^ Rostellaria with long spinous processes, 

 found by myself in M'^Nairy county, West Tennessee, and described 

 by Troost under the name of i?. macrodactyla ; two species oi Denta- 

 lium; diHamite found by myself in the greensand in M*^Nairy county 

 and described by Troost as H, Verneuili ; Hamites Leai, Troost ; 

 Baculites, species undetermined. No minerals have, I believe, been 

 observed in the beds of marl and greensand ; but on the west bank 

 of the Tennessee river I examined an arenaceous deposit, which 

 Troost considered as immediately overlying these strata; and it con- 

 tained, besides exogenous trees converted into flint, lignite, pyrites 

 and retinasphalt. 



Two distinct and very extensive coal-formations are included in 

 the territory I am describing, as may be seen by reference to the 

 mapf. 



That on the west is the Great Illinois Coal-field, laid down and 

 defined in my Report on Iowa, Wisconsin and Northern Illinois. It 

 equals in area the entire island of Great Britain, extending from 

 S.E. to N.W., from the waters of Oil Creek and Rome on the Ohio 

 river to the mouth of Rock river on the Mississippi, a distance of 

 300 miles; from south to north, from the waters of Green River 

 ■ and Tradewater in Kentucky to the waters of Little Vermilion in 



* See Map, PI. XIX,, where the marl is inserted on Dr. Troost's authority. 



t These coal-fields have since been described at length, and with reference to the 

 other American deposits of the same kind, by j\Ir. Lyell, in his recent work on 

 America. 



