566 N. H. DARTON FISH REMAINS IN ORDOVICIAN ROCKS 



latitude 45 degrees, was subjected to erosion during Silurian and later 

 times, which may have resulted in the removal of Ordovician rocks in all 

 the area in which they are now absent. At no locality has there been 

 observed evidence of an original margin of the later Ordovician, either 

 by shore deposits or conformable overlap of deposits of the next succeed- 

 ing system (Silurian). The thinning and absence of the Ordovician 

 rocks and overlap by Carboniferous rocks is in itself no evidence at all, 

 because a great thickness of sediments could have been removed in 

 Silurian-Devonian times, especially along zones of increased uplift. The 

 thinning out of the Bighorn formation in the southeastern portion of the 

 Bighorn mountains apparently is due to the latter cause. 



From the foregoing it will be seen that the various maps which have 

 been prepared showing "land areas in Ordovician" times are misleading 

 in the region to which this paper relates. Evidently in much of the 

 region described there were land surfaces in early Ordovician (pre- 

 Trenton) time, for the sandstones underlying the Bighorn and Fremont 

 limestones usually lie unconformably on earlier deposits and show shore- 

 line features. They are widely overlapped by the succeeding limestones, 

 the products of deeper water deposition, but the extent of this deep water 

 is an unsolved problem, for no evidences of its shores have been found. 

 In the central Colorado region at least, the sandstone (Harding) conform- 

 ably overlies and in places overlaps a somewhat earlier Ordovician lime- 

 stone (Manitou), which indicates that in places there were deeper or 

 quieter waters preceding those which deposited the sandstone. 



It appears probable that the Ordovician rocks west of Colorado Springs 

 and near Canyon City are projections from an extensive area underlying 

 the plains region eastward, as suggested in figure 6. If this exists, its 

 western edge has been eroded and buried beneath the Eed Beds-Granite 

 overlap, excepting in the old embayments where protected from erosion. 



We have but few data as to the rocks and geologic history of the later 

 Ordovician. The limestones which represent the Eichmond occupy an 

 area of considerable size in the central and northern portions of the Big- 

 horn mountains, and are separated from the underlying limestone of 

 Trenton age by a hiatus representing a long interval of time. They may 

 have been deposited extensively in the Northwest and later mostly re- 

 moved by the widespread post- Ordovician erosion, or, on the other hand, 

 may have been only laid down in restricted basins. Apparently all of the 

 area treated in this paper was a land surface during the interval between 

 Trenton and Eichmond times, although there may have been some local 

 areas of deposition in which the rocks now are buried, or from which they 

 may have been removed, prior to Eichmond deposition. 



