PALEOZOIC STRATIGRAPHY OF ROCKY MOUNTAINS 391 



preceding ones, was accomplished, so far as the known record 

 shows, without crustal deformation. In Wyoming, Montana, and 

 Colorado, this interval of emergence may have continued without 

 interruption until Devonian time. 



The Niagaran{?) invasion. — Toward the middle of the period, 

 probably in the Niagaran epoch, the sea invaded this region again, 

 from the Great Basin side. The sediments deposited in this sea 

 are known only in northern Utah (and in Nevada ?) , but may once 

 have been far more extensive (see Fig. 10). The Conchidium 

 knighti fauna found in the Laketown dolomite implies a marine 

 connection with Alaska and with Europe. 



DEVONIAN 



Pre- Jefferson erosion. — The Jefferson dolomite overlaps far 

 beyond the Laketown dolomite into western Wyoming and south- 

 western Montana. In Wyoming it rests upon various horizons 

 of the Upper Bighorn dolomite. In Montana, north and west of 

 Livingston, it lies directly upon the Upper Cambrian. It is prob- 

 able that the Bighorn dolomite once extended farther into Montana, 

 and was removed by pre-Jefferson erosion. If the Silurian system 

 originally extended into Wyoming or Montana, it also must have 

 been removed at this time. In Utah, no evidence of a hiatus 

 between the Silurian and the Devonian has been noted. 



Although the Jefferson dolomite overlaps both the Silurian and 

 the Ordovician systems, the surface upon which the first sediments 

 of the Jefferson dolomite were deposited appears to have been 

 almost as level as that on which the Lower Bighorn dolomite was 

 laid down. A suggestion of relief is furnished by the contrast 

 between the sections at Logan and Livingston, Montana. Near 

 Logan, the Jefferson rests upon Member 3 of the Cambrian system. 

 At Livingston Peak, about 50 miles to the east, 438 feet of Bighorn 

 dolomite intervene between the Jefferson and that member. Even 

 this difference indicates but an inappreciable slope. 



Devonian marine invasions. — The Devonian submergence, dur- 

 ing which the Jefferson dolomite was deposited, was probably 

 interrupted in Wyoming and Montana by at least one interval of 

 emergence. The sea appears to have made two or three successive 



