8 



GE^OIvOGY OF IvA SALIvE COUNTY. 



tion, but have been tilted and bent until they lie at 

 all possible ang'Jes, and are, in some cases, even over- 

 turned, so that the lower strata are uppermost. In 

 this region, however, they are but sliofhtly inclined in 

 g-eneral, the only exception being- in the country border- 

 ing the Big Vermillion and in the west part of the 

 county. 



Rocks whose strata are nearly parallel to each 

 other are said to be conformable; those whose beds 

 are at an angle one with another unconformable. Un- 

 conformability in the rocks of the Central Plain is not . 

 common. One example exists in the carboniferous 

 sandstones, in the hill where the road ascends the blulf 

 beyond the stone bridge over Covell creek on the river 

 road. Along the Big Vermillion they are inclined at 

 high angles, 30 ^ to 45 ^ , and much flexed and broken, 

 but in other places depart but a little from hori- 

 ^ontalitv, and lie, it may be assumed, in nearly the 

 position in which they were deposited, and others at 

 the tunnel and near the Big Vermillion river, as we 

 shall see farther on. Our knowledge of the rocks 

 lying far below the surface is derived from the walls 

 of ravines cut into them by streams, from the places 

 where by tilting the}^ have been brought to the surface, 

 and from mines and borings, but the deepest mines are 

 less than four-fifths of a mile, and the deepest borings 

 but little more than a mile deep, and ihese distances 

 are very small compared with the total thickness of the 

 rocks at any one point, but strata are often so inclined 

 that the edges of many thick beds are exposed, and it 

 is from such exposures that we acquire most of our 

 knowledge of the deeper lying beds. 



We have said abo /e that the strata were origin- 

 ally nearly horizontal, and also that they were depos- 

 ited along coast lines or in inland seas or lakes. They 



