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Bordering this platform in the south-western part of Clarke county 

 would be seen the next step in the series, that composed of the 

 Clinton limestone, much lower and narrower than the others, and of 

 very irregular outline. Finally, spread over the rest of the region 

 indicated, would appear a floor of Blue limestone, hidden from view 

 here and there by patches or islands of the layers already mentioned, 

 some of these islands being composed of the remains of the next 

 higher step, some of the two preceding ones, but none showing any 

 traces of the highest or Helderberg formation. The whole surface 

 would appear scarred and seamed by erosive agencies, some of which 

 are now in operation and some of which have long ceased to exist. 

 Let us now suppose this series of steps to be inclined somewhat 

 toward the nprth-east so that their outer edges rise to nearly the same 

 level and there inner angles form a series of shallow troughs. The 

 long axis of these troughs extends generally from the north-west to 

 south-east and Mad river traverses them nearly at right angles, its 

 general trend being from north-east to south-west. The edge of 

 the Helderberg step is crossed in the northern part of Cham- 

 paign county as we have already seen. After this the river is 

 bedded in the overlying drift deposits with which the whole trough 

 at the foot of the Helderberg layer is filled. Of this basin and its 

 marshy character we have already treated. The next step reaches 

 the surface in the neighborhood of Springfield, that of the Niagara 

 limestone. Through the edge of this the river has worn a narrow 

 gorge for some distance. The picturesque cliffs at Moore's and 

 Holcomb's quarries, nearly one hundred feet in height, are good 

 examples of this erosion. Here the river valley is suddenly nar- 

 rowed, its width opposite Moore's quarries being not more than one- 

 quarter of a mile. Before this rocky barrier was worn away the 

 river may have had a considerable fall at this place, the rock 

 structure being precisely like that at Niagara Falls, and continuous 

 with it. We may also suppose the marshy region behind to have 

 been occupied by a lake or series of lakes, before the present free 

 outlet for the waters was afforded. At Springfied, Buck creek en- 

 ters Mad river from the east, and two miles below the city Mill 

 creek. These latter have also wrought the latter part of their 

 channels through the Niagara limestone. Five miles below Spring- 

 field, at Snyder's station, the river traverses the next layer of the 

 series, the Clinton limestone. The valley now rapidly broadens and 



