Handbook of Paleontology 307 



ticosti in the Gulf of St Lawrence; and the middle and 

 southern Appalachian area. Silurian rocks cover large 

 areas on the south shore of Hudson bay. In the east- 

 ern part of North America Silurian beds are found 

 resting unconformably upon folded and eroded Ordo- 

 vician strata ; in the middle states unconformably upon 

 Ordovician or older rocks where the Silurian seas over- 

 lapped the area covered by the Ordovician seas. In 

 the western region (Montana, Utah) the strata of Ordo- 

 vician, Silurian and Devonian seem to be conform- 

 able, thus falsely suggesting unbroken deposition in 

 this area. Silurian rocks are far thicker in the east 

 than in the west or in the interior, especially along the 

 Appalachian range. The thickness of the limestones 

 in the west and the interior are given above, also the 

 thickness of the Appalachian trough deposits in Penn- 

 sylvania (mainly elastics, 6500 feet), from which area 

 the Silurian materials thin rapidly to the south and 

 north. The Silurian is widespread in the Acadian 

 trough where 4000 feet of shales and sandy limestones 

 occur. Farther south at Black Cape, Bay of Chaleur r 

 Quebec, 7000 feet of deposits occur, here terminated 

 by lava flows. Translated into terms of limestone de- 

 posits, Silurian deposits are the equivalent of 3000 to 

 4500 feet of limestone. • 



The Silurian was a quiet period. Volcanic activity 

 was rare (southern Maine) and only in a few places 

 do igneous intrusions occur in North America. Here 

 the close of the period is marked by no disturbance 

 and it is generally believed that deposition continued 

 uninterruptedly into the Lower Devonian, though in 

 certain areas unconformities have been noted. In 

 Europe and other continents mountain building marked 



