﻿IOI CORRRELATION 23 



supports the view that the Jefferson limestone represents both 

 Lower and Middle Devonian. No evidence of a stratigraphic 

 break between the Jefferson and the preceding formation has 

 been observed by the writer in any of the sections examined by 

 him. None of the folios 26 which describe the Jefferson limestone 

 record any unconformity at its base. Peale 27 found no proof of a 

 Silurian fauna in the Threeforks region, but considered sedi- 

 mentation continuous from the Cambrian to the Devonian. He 

 expressed the belief that "we need not be surprised if some time 

 in the future this now barren interval should somewhere furnish 

 a mingling of Devonian and Silurian forms 28 ." In Utah the 

 writer has found conditions that seem to indicate there the con- 

 tinuity of sedimentation from the Silurian to the Devonian, 

 which although probable in Montana has not been demonstrated. 

 In the sections east of Cache Valley a Silurian fauna occurs in a 

 white magnesian limestone 29 ' 200 feet to 300 feet thick. The* 

 much darker colored magnesian limestones (Jefferson limestone) 

 which lie above the Silurian limestone contain an abundance of 

 Devonian fossils in certain beds in the lower part of the formation. 

 Martinia maia is the most abundant of these. Besides the 

 Devonian fauna, Haly sites catenulatus occurs rather commonly in 

 the lower portion of the Jefferson limestone and is present also in 

 the Silurian fauna below. Its occurrence in the Jefferson is the 

 result, it seems most probable, of continuous marine conditions 

 from Silurian to Devonian time, which permitted Halysites to con- 

 tinue into the post-Silurian period as an associate of the Devonian 

 fauna. Halysites catenulatus was considered a diagnostic Silurian 

 fossil until it was found associated with Ordovician faunas some 

 years ago. The section under consideration shows that in Utah 

 it ranges upward into the Devonian, and is apparently the first in 

 which a post-Silurian horizon has been recorded for this species. 



^Geologic Atlas U. S., folios, 1, 1894; 24,1896; 30, 1896; 52, 1899, 

 U. S. Geol. Survey. 



s7 Geologic Atlas U. S., folio, 24, 1896, U. S. Geol. Survey, 1896, p. 2. 



^Ibid, p. 2. 



^Kindle, E. M., "Occurrence of the Silurian fauna in Western 

 America." Am. Jour. Sci., vol. 25, 1908, p. 127. 



