TERRIBLE FISHES AND THEIR COMPANIONS. 179 



the Devonian strata disappear they go under the Carbonifer- 

 ous strata ; and these go under all the newer strata which 

 may be present. Remember, however, as before said, that a 

 whole formation may be found missing in particular places. 

 Strata were deposited only where sea-bottom existed. If the 

 spot was uplifted so as to be dry land during a particular 

 age, the formation belonging to that place can not exist. 

 But if the spot became sea-bottom in the next age, the for- 

 mation belonging there was deposited, and it does not now 

 lie on the formation of next preceding age, but on the one 

 before that. 



So do not imagine ourselves penetrating deeper and deeper 

 into the earth. We examine the systems of strata in the re- 

 gions where they come to the surface. We may presume 

 they continue under the newer formations to great depths ; but 

 I have the opinion that if we could follow them, they would be 

 found gradually growing thinner. 



Let us begin by learning where the Devonian strata occupy 

 the surface. Nowhere in New England are they distinctly 

 revealed. Nor in any of the Gulf States. A belt of Devon- 

 ian strata stretches east and west through central and south- 

 ern New York, from the Helderberg Mountains to Lake Erie. 

 Thence it passes under Lake Erie and along both shores to 

 the extremity of the lake, and into south-eastern Michigan. 

 Here the outcrop divides; one branch passes south, through 

 the west center of Ohio to the Ohio River, and the other., 

 turning north, goes under Lake Huron and along its western 

 border to the Straits of Mackinac. This branch here bends 

 westward and south-westward, so as to underlie the central 

 and eastern part of Lake Michigan, and border that lake on 

 the east. This branch goes down through Indiana to the Ohio 

 River, at the Falls of the Ohio, lying along the eastern border 

 of the great Coal Field of Indiana, Illinois, and Kentucky. A 

 belt also extends from Rock Island, Illinois, north-westward 

 by Iowa City, through the state of Iowa. This system is found 

 also in Missouri, Kentucky, Tennessee, and other states. 



There is a very useful key to the distribution of the rocks 



