310 OK THE FORMATION OF KCCKS. [July, 



(luced by submarine eruptions, tlioui^li the periods of 

 their formation were long before the date of our records. 

 It is under such circumstances that the alternation of 

 the Neptunian and Volcanic formations most frequent- 

 ly takes place, as in the islands of Dominiqua, St. 

 Christopher, and St. Eusiatia, in the West Indies; (he 

 iish and shells found in the lava in the Vincentin, &c. 

 so that the finding of abed of shell limestone between 

 two currents of lava, would be agreeable to the laws of 

 nature, and no deviation from the common order of 

 things. 



{To be continued.) 



Observations on a new Genus of Fossil Shells* 

 By C. A. Le Sueur. Read June 30, 1818. 



The secondary blue limestone of wliich the great 

 basin is composed, which extends from tlie Alleghany 

 Mountains to Lake Superior, and from Saratoga to 

 the Mississippi, includes numerous fossil shells, 

 which, in some of the strata, are almost exclusively of 

 a single species of Terebratula ; other species of this 

 genus, equally numerous in individuals, form other 

 strata, mixed with Encrinites, Alcyonites, Caryophil- 

 lites, Favosites, Gyrogonites, kc In another stratum 

 is found the Alcyonite, Trilobite, a Terebratula with 

 flat valves, a Favosite, kc. ; and a large discoi-. 

 dal shell, %vhich more particularly forms the sub- 

 ject of this paper, and which at fiigt sight resemble* 



