76 WALKS AND TALKS. 



Mackinac and Cheboygan. Mackinac Island, which we have 

 talked about, is of Corniferous Limestone. The eastern mar- 

 gin of this dish is at London, in Ontario, and the western is 

 under Lake Michigan. A little nearer the center of the state 

 we find the margin of the next overlying group the Hamil- 

 ton. So the other groups follow as the successively smaller 

 dishes. The top dish consists of the Coal Measures. It is a 

 pretty flat dish, however, since the middle is about as high 

 as the margin. 



That is one kind of arrangement which we observe. More 

 frequently, however, the arrangement is more like a pile of 

 long, broad, thin troughs without ends. In this case you per- 

 ceive that each formation goes down on one side and comes 

 up on the opposite side. But at the ends, they may not ap- 

 pear at the surface. Such an arrangemeut of strata is 

 called synclinal, and the line along the middle is the 

 synclinal axis. 



Still another arrangement is quite as common. Suppose 

 we turn our nest of wooden dishes upside down, and suppose 

 that is their natural position. They represent so many for- 

 mations still. Then suppose we saw through the nest hori- 

 zontally in such place as to saw off all the bottoms except that 

 of the smallest dish. It is done. Now you see the edges ot 

 the formations presenting themselves in concentric outcrops. 

 Plant yourself on the inner or middle formation. Notice that 

 this now underlies all the others. This also, dips toward all 

 the others, and passes under them out of sight. Then fix 

 your attention on the outcrop of the next dish. Notice that 

 this dips away from the first one the older one, first laid 

 down ; and that it dips toward the newer or overlying forma- 

 tions. And so to the uppermost or newest each netver dip- 

 ping away from all the older. 



This arrangement of strata is common. The city of Cin- 

 cinnati stands on an upward bulge of Cambrian rocks. All 

 around, at the distance of some miles, may be seen the 

 outcropping edge of the Silurian System. The Silurian strata 

 overlie the Cambrian, as shown in the Table, page 73, and 



