386 PALAEOZOIC TIME. 



Fragmental rocks. Limestones. 



1. Potsdam period 6,800 200 



2. Rest of Lower Silurian 1,600 6,000 



3. Lower Silurian era 8,400 6,200 



4. Upper Silurian era 6,760 600 



5. Devonian Age 14,300 100 



6. Carboniferous Age 14,600 125 



Limestones increase with extreme slowness, as explained in the 

 chapter on coral islands. From five to ten feet of fragmental de- 

 posits will accumulate while one of limestone is forming. This con- 

 clusion is sustained by the ratio in any given period between the 

 fragmental rocks of the Appalachians and the limestones of the 

 Interior basin. 



Taking the ratio as 5 to 1, and making the substitution accord- 

 ingly, the numbers are, respectively, (1) 7800; (2) 31,600; (3)39,400; 

 (4) 9760; (5) 14,800; (6) 15,225. These numbers have nearly the 

 ratio 1:4:5:1^:2:2. Hence, for the Silurian, Devonian, and Car- 

 boniferous ages, the relative duration will be 6| : 2 : 2, or not far 

 from 3:1:1. If the ratio 8 to 1 be taken, these numbers become 

 4:1:1. 



According to these calculations, the American Lower Silurian era 

 was, by the first estimate, four times as long as the Upper ; by the 

 second, five times ; and the duration of the Silurian age was three or 

 four times that of either the Devonian or the Carboniferous. The 

 earth thus dragged slowly on through its earliest periods. 



II. American Geography. 



1. General course of progress. — Through the Palaeozoic ages, the 

 dry land of the closing Azoic age (map on p. 136) gradually ex- 

 tended southeastward, southward, and southwestward. At the end 

 of the Silurian, the limit of the dry land appears to have had its 

 position near the central east-and-west line of the State of New 

 York ; and at the close of the Devonian it lay not far from the 

 southern border of the State. Westward, beyond Michigan, in Illi- 

 nois, Iowa, and Minnesota, there was a like expansion to the south 

 and west of the Wisconsin Azoic. Michigan long continued to be 

 a part of the oscillating Interior basin, the Palaeozoic formations 

 being continued there even to the close of the Coal period. 



Along the St. Lawrence the Ottawa basin was nearly obliterated 

 at the close of the Lower Silurian (p. 228). In the latter half of the 

 Upper Silurian the river opened into a St. Lawrence gulf over the 

 site of Montreal, and in its waters a Lower Helderberg limestone 





