XV.] CAMBRIAN AND SILURIAN IN NORTH AMERICA. 415 



with layers of shales, the whole representing a period of dis- 

 turbance which probably corresponds to the almost complete 

 paleontological break between the magnesian limestones of the 

 Calciferous and the pure limestones of the Chazy period.] 



The Levis and Chazy formations, as we have seen, offer a 

 commingling of forms of the first and second faunas, which 

 shows them to belong to a period of transition between the 

 two ; but it is remarkable that, so far as yet observed, no rep- 

 resentatives of the latter of these faunas are known to the 

 east and south of the Appalachians, along the Atlantic coast ; 

 the first fauna, whether in Massachusetts, New Brunswick, 

 or southeastern Newfoundland, being unaccompanied by any 

 forms of the second. The third fauna, on the contrary, is 

 represented in various localities both within and to the east of 

 the Appalachian region, from Massachusetts to Newfoundland. 

 In parts of Gaspe, and also in Nova Scotia, strata holding 

 forms referred to the Clinton and Niagara divisions are met 

 with, as well as other strata, of Lower Helderberg age, asso- 

 ciated with species of shells and of plants which connect this 

 fauna with that of the succeeding Lower Devonian or Erian 

 period. To this Lower Helderberg horizon (corresponding to 

 the Ludlow of England) appear to belong certain fossiliferous 

 beds found along the Atlantic coast of Maine and of New 

 Brunswick, in Nova Scotia, and probably in Newfoundland ; 

 as well as others included in the Appalachian belt in Massa- 

 chusetts, New Hampshire, Vermont, and Quebec, along the 

 Connecticut valley and its northeastern prolongation. The 

 fossiliferous strata just noticed, both in the Connecticut valley 

 and along the Atlantic coast, occur in small areas among the 

 older crystalline schists, often made up of the ruins of these, 

 and in highly inclined attitudes. The same is true in other 

 places of the similarly situated strata of Cambrian, Devonian, 

 and Lower Carboniferous periods. These derived strata, of 

 different ages, have, from their lithological resemblances to the 

 parent rocks, been looked upon as examples of a subsequent 

 alteration of palaeozoic sediments ; and by a further extension 

 of this notion, the pre-Cambrian crystalline schists themselves 



