696 STUART WELLER 
connecting the two regions has never been definitely located. 
During Ordovician time there was an open passageway through 
the St. Lawrence valley joining the interior epicontinental sea 
with the ancient Atlantic Ocean, but during the Ordovico-Silurian 
transition period, the Taconic range of mountains was elevated, 
and this passage entirely closed. The Appalachian land was 
the eastern barrier to the interior Silurian sea, and during this 
period this land was joined to the Laurentian land at the north. 
East of this land barrier the Silurian fauna occurs in the eastern 
provinces of Canada, but these eastern strata are not continuous 
with those in New York, and the communication between the 
two regions was not direct. 
In a southern direction the Silurian strata thin out and 
become more clastic in constitution, indicating proximity to a 
shore line, and it is probable that even at this early period the 
western extension of the Appalachian land, described by Gris- 
wold* and Branner? was already in existence. 
The western extension of Silurian strata cannot be definitely 
shown, but they are nowhere a conspicuous feature in the United 
States further west than Iowa. Beds in the far West containing 
the chain coral, Halysztes, have been referred to this period, but 
usually upon insufficient evidence, for this genus is known also to 
occurin the Ordovician. In those rare instances where other forms 
have been found associated with the chain coral, they usually 
have been of an Ordoyician rather than a Silurian facies. 
Nowhere in the great western region has the wonderfully prolific 
Silurian fauna of the East been found, and it is safe to assume 
that the greater part of this region was above sea level during 
Silurian time. 
This leaves the North as the only available outlet for the 
interior Silurian epicontinental sea. A glance at the accompany- 
ing map (Fig. 1) indicating the distribution of the Silurian outcrops 
in North America, shows their northward extension. There is 
an extensive area in the region of Lake Winnipeg (XIII), one 
‘Proc. Bost. Soc. Nat. Hist., Vol. XXVI, p. 474. 
2 Am. Jour. Sci. (4), Vol. IV, p. 357. 
