THE TERTIARY 135 



in non-glaciated tracts, and it is known to reach the thickness sometimes 

 of 100 feet, even in areas of cr3'3talline rocks* 



Length of the Tertiary Age 



That this pre- Glacial epoch of luxuriant life, which was destined to 

 be rudely terminated, was one of long duration may be inferred from 

 one or two considerations. 



First, the great mammals which characterize the Tertiar}^ had no 

 known representatives in the Cretaceous ; hence the time of the Tertiary 

 w^as sufficiently long for their evolution from reptilian or other forms, as 

 well as for their spread and decline. Their extermination seems to have 

 been gradual, some of them having survived even through the Glacial 

 period and almost into the period of human history. As a group, how- 

 ever, the great mammals ceased with the close of the Tertiary. As a 

 fauna the Tertiary land animals constitute at once the most remarkable 

 and the highest form of all the extinct types of life that have dwelt on 

 the globe. The time required for the development of such a dynasty 

 must have been long, its years counting up into the hundreds of thou- 

 sands or perhaps millions. 



A second means of roughly estimating the length of time involved in 

 the Tertiary is by comparison of the erosion ejBfected by streams. It is 

 well known that within post-Glacial time the rivers of North America 

 within the last glaciated latitudes, whether they flow^ over the drift de- 

 posits or over the rocks in situ, have done so little eroding that the val- 

 le3's seem to have been born but yesterday. The marks of the ancient 

 glaciers on the striated surfaces are hardly obliterated, and in northern 

 Minnesota no perceptible erosion of the cr3-stalline rocks can be made 

 out; except where the streams have been aided by local and accidental 

 conditions. The retreat of waterfalls and the production of river gorges 

 since the Glacial epoch are due to favorable stratification. 



If we compare this lack of erosion in the drifted latitudes with that 

 which was effected by streams within Tertiary time, w^e are impressed 

 by the great contrast. In Colomdo and in many other portions of the 

 Rocky Mountain region are granites whose origin dates from some point 

 in Tertiary time, and these granites are rotted to gravel and to soil. 

 Streams that flow over them have cut deep gorges and canyons. The 

 weather has wrought them into fantastic pinnacles or mural cliffs whose 

 silent voices tell of their hoary antiquity. Of this decay and erosion 

 but little can be laid at the door of post-Glacial time. The years since 

 the retreat of the ice from Minnesota can be counted in thousands on 



*R. Pumpelly : Am. Jour. Sci., vol. xviii, .\ugust, 1879. 



