GENERAL OBSERVATIONS. 1023 



comprehensive were the agencies set to work : glaciers that reached across 

 from ocean to ocean ; rivers deriving magnitude and energy from the loftiest 

 of mountains, the greatest of ice-sheets and the most abundant of rains ; 

 and a genial climate that reached almost to polar latitudes, and produced 

 luxuriant growth in all life, animal as well as vegetable. Thus were evolved, 

 as never before, the sublimity of the mountain peaks, and the richness and 

 varied beauty of the valleys and wide-reaching plains, and the many other 

 surface details that were essential to the pastoral and agricultural pursuits 

 with which man was to commence his own development. 



GENERAL OBSERVATIOXS OX GEOLOGICAL HISTORY. 

 LENGTH OF GEOLOGICAL TIME. 



Time-ratios. — In the preceding edition of this work estimates are given 

 of the relative lengths of the ages and periods, or their time-ratios, based on 

 the maximum thicknesses of the rock formations of the several periods, 

 allowing a ratio of 1 to 5 between the rate for limestone and that for ordi- 

 nary fragmental rocks. These thicknesses have since been increased much 

 for some parts of the geological column ; but the increase is not far from 

 proportional to the former numbers. The evidence at present obtained ap- 

 pears to favor the conclusion that the relative duration of the Cambrian and 

 Silurian, the Devonian and the Carboniferous eras, corresponds to the ratio 

 4|- : 1 : 1, or perhaps 4:1:1, the ratio hitherto adopted ; and for the Paleo- 

 zoic, Mesozoic, and Cenozoic, 12 : 3 : 1. The thickness of upturned rocks 

 is so difficult to obtain with accuracy, and is so certainly exaggerated 

 greatly when the absence of faults and flexures is not ascertained before 

 drawing conclusions, especially in connection with the older tilted forma- 

 tions, that much careful geological work is yet to be done before reliable 

 ratios can be deduced. 



Length of time since the Glacial period. — The facts with regard to the 

 present rate of recession of Niagara Falls have been used for calciilating the 

 length of time since the Glacial period. The argument has been presented 

 thus. Niagara has made its present gorge by a slow process of excavation, 

 and is still prolonging it toward Lake Erie. Near the fall, the gorge is 200 

 to 250 feet deep, and 160 feet at the fall, — the lower 80 feet shale, the upper 

 80 limestone. The waters wear out the shale, and thus undermine the lime- 

 stone. The rocks dip 15 feet in a mile up stream, so that the limestone at 

 the fall becomes thicker, as retrocession goes on. The distance from Niagara 

 to the Queenston Heights, which face the plain bordering Lake Ontario, is 

 seven miles. The general features of the region are shown in the bird's-eye 

 view, page 973. The new excavation began again at the Queenston Heights, 

 and gradually extended southward to its present limit at the Falls. The 

 time of beginning was after the filling of the channel with drift, which 

 occurred during the retreat of the glacier. The rate of progress in the exca- 



