CHAPTER IV. 



THE LOWEE AND UPPER SILURIAN AOES. 



BY English geologists, the great series of formations 

 which succeeds to the Cambrian is usually included 

 under the name Silurian System, first proposed by 

 Sir Roderick Murchison. It certainly, however, con- 

 sists of two distinct groups, holding the second and 

 third faunas of Barrande. The older of the two, 

 usually called the Lower Silurian, is the Upper 

 Cambrian of Sedgwick, and may properly be called 

 the Siluro- Cambrian. The newer is the true Silurian, 

 or Silurian proper the Upper Silurian of Murchison. 

 We shall in this chapter, for convenience, consider 

 both in connection, using occasionally the term Lower 

 Silurian as equivalent to Siluro- Cambrian. The Silu- 

 rian presents us with a definite physical geography, 

 for the northern hemisphere at least ; and this physical 

 geography is a key to the life conditions of the time. 

 The North American continent, from its great un- 

 broken area, affords, as usual, the best means of 

 appreciating this. In this period the northern cur- 

 rents, acting perhaps in harmony with old Laurentian 

 outcrops, had deposited in the sea two long submarine 

 ridges, running to the southward from the extreme 

 ends of the Laurentian nucleus, and constituting the 

 foundations of the present ridges of the Roeky 



