128 E. M. Kindle — Silurian Eaun a in Western America. 



brachiopod, which is one of the most common and widely 

 distributed Silurian species in the Eastern States, places the 

 Silurian age of this fauna beyond question. 



This fauna occurs in a light-colored magnesian limestone in 

 which local developments of siliceous or quartzitic lenses 

 occur. The fossils were obtained from one of these siliceous 

 beds. The same fauna was seen in Logan Canyon, 12 miles to 

 the north of the locality from which the collection was made. 

 The magnesian limestone formation holding this fauna has a 

 thickness of from 200 to 300 feet. Below the Silurian lime- 

 stone is a limestone of much darker color and of undetermined 

 age. Above the Silurian limestone lies a series of dark mag- 

 nesian limestones 800 to 1000 feet thick, carrying a Devonian 

 fauna. A band of thin-bedded or laminated limestone in 

 which no fossils have been found separates the dark magne- 

 sian limestones from the Silurian beds in some of the sections, 

 while in others it is absent. 



The three localities which have been cited are widely sepa- 

 rated from one another, but it is most probable that numerous 

 other occurrences of the Silurian fauna will be found in the 

 intervening areas.* The absence of the Silurian from many 

 of the carefully studded sections in the Front Range region 

 of the Rockies would seem to justify the prevailing view that 

 a land area occupied that region in Silurian times. What the 

 extent of that area may have been it is hardly profitable to 

 guess with our present knowledge. That it occupied much of 

 the present area of Colorado is most probable. It seems to 

 have been bordered hj a Silurian sea on the west and south- 

 west. 



The occurrence of a Silurian fauna in northern Alaska has 

 a direct bearing on one of the interesting problems of Silurian 

 paleogeography, the route of intermigration between the Silu- 

 rian faunas of Europe and America. The composition of the 

 Porcupine River fauna indicates quite clearly that it repre- 

 sents the general Silurian fauna of Europe and the interior of 

 America. 



Professor Weller has attempted to point out the path of 

 migration by which the cosmopolitan Middle Silurian fauna 

 reached eastern America from northern Europe, where it is so 

 well developed. The hypothetical western shore line of the 

 Silurian sea as drawn by Weller runs northward of Arkansas, 

 slightly to the west of the Mississippi in the United States, 



*Diller has found a Silurian fauna in California (Bull. Geol. Soc. Am., 

 vol. iii, p. 376, 1892, and Bull. U. S. Geol. Survey No. 323, 1907). In New 

 Mexico Gordon and Graton have reported Silurian rocks at Silver City (this 

 Journal (4), vol. xxi, p. 394, 1906). Bichardson reports the same fauna at 

 El Paso, Tex. (Univ. Texas Min. Surv., Bull. No. 9, p. 31, 1904). 



