166 PHYSICAL GEOGRAPHY. 



From the Apalachian chain to the Rocky Mountains 

 the fundamental rocks would appear to be silurian* over- 

 laid in large tracts by the old red sandstone and the car- 

 central area of the river generally exceeded three miles an hour, and 

 was less towards the sides. — Proceed. Amer. Assoc, p. 340. 



* As the silurian rocks are most extensively developed in North 

 America, and are of constant recurrence throughout the route of the 

 Expedition, I shall introduce here, for reference, a tabular enume- 

 ration in the order of their superposition, as ascertained by the United 

 States geologists, beginning with the lowest. 



A. Champjlain Division ; supposed to underlie three-fourths of the 



territory of the United States, and to occupy the surface of one 

 half. 



1 . Potsdam sandstone ; a quartzose rock, generally grey, often 

 striped, sometimes partially or wholly red : is supposed to be the 

 lowest sedimentary fossiliferous rock. The Taconic rocks of 

 Dr. Emmons, consisting of lamellar white limestone, with specular 

 iron ore, are supposed to be the earliest deposits of this period, 

 modified by metamorphic agency. Footmarks of reptiles have 

 been found by Mr. Hunt in this sandstone. 



2. Calciferous sandrock ; a deposit of calcareous and earthy matter 

 variously mixed. 



3. Black river or Chazy and Birds-eye limestone ; a brittle limestone, 

 having a smooth, flat-conchoidal fracture. This limestone, cha- 

 racterised by its peculiar fossils, is of frequent recurrence north- 

 wards, up to the islands of the Arctic Sea. 



4. Trenton limestone; a dark-coloured limestone, interlaminated 

 with dark shaly matter. Often metalliferous. 



5. Utica slate; dark-coloured carbonaceous slates, which readily 

 disintegrate. 



6. Hudson River group, Loraine shales, Frankfort slate, and Rubble- 

 stone ; mostly dark slates and shales, and grey thick-bedded grit 

 stones. 



B. Ontario Division, includes a series of limestones, shales, and 



sandstones, which pass insensibly into each other. 



7. Grey sandstone of Oswego. 



8. Medina sandstone; red or slightly variegated, and of every 

 degree of coherence. Originates many brine springs. 



9. Oneida conglomerate; a variable intermixture of sand and quartz 

 pebbles. 



10. Clinton group ; consisting of deposits of various characters, 



