mental rocks. 



Limestones 



7,000 



200 



18,000 



6.000 



25,000 



6,200 



6,760 



600 



14,300 



100 



16,000 



125 



GENERAL OBSERVATIONS. 381 



the series of rocks pertaining to the several ages affords some data for 

 estimating their time-ratios. The results are necessarily uncertain, 

 since the increase of a rock is often directly connected with the subsi- 

 dence there in progress, as has been elsewhere explained. Still, the 

 conclusions are sufficiently reliable to be here presented. 



Taking the maximum thickness, along the Appalachians, of the suc- 

 cessive formations (the limestone and fragmental beds in each case 

 from the same region), we find for the 



1. Potsdam period . . . 



2. Rest of Lower Silurian 



3. Lower Silurian era . , 



4. Upper Silurian era 



5. Devonian Age . . . 



6. Carboniferous Age . , 



Limestones increase with extreme slowness, as explained in the 

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

 will accumulate while one of limestone is forming. This conclusion 

 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 accordingly, 

 the numbers are, respectively, (1) 8,000; (2) 48,000; (3) 56,000; 

 (4) 9,760; (5) 14,800; (6) 16,625. These numbers have nearly the 

 ratio 1:6:7:1^:2: 2. Hence, for the Silurian, Devonian, and Car- 

 boniferous ages, the relative duration will be 8^- : 2 : 2, or not far from 

 4 : 1:1. Or, the Silurian age was four times as long as either the De- 

 vonian or Carboniferous ; and the Lower Silurian era nearly six times 

 as long as the Upper Silurian. 



In the Silurian age, the ocean worked almost alone, in the wear and 

 accumulation of rock material, while in the Carboniferous, at least 

 about Nova Scotia, where the Carboniferous rocks are nearly three 

 times as thick as elsewhere, river-action aided greatly in the result. 

 Hence the ratio 4:1:1 would seem to give the relative length of the 

 Carboniferous age too high. Yet, as the eras of the several coal beds 

 must have been each of great length, the ratio can hardly need 

 change on tins account. 



H. Life. 



1. System of progress. — The Animal kingdom began with Proto- 

 zoans, then followed Radiates, Mollusks, and water- Articulates ; it 

 included Fishes, the lower Vertebrates, in the closing Silurian ; and 

 Amphibian species in the commencing Carboniferous age. With 

 each period, the progress was upward, toward a fuller and higher dis- 

 play of the system of life, though not beginning always in the lowest 

 species of a group. 



