HOW OLD IS THE EARTH? 161 



Robert Bell, where veins of quartz marked with glacial strise 

 stand out to various heights not exceeding one foot above the 

 weathered surface of the inclosing limestone. 



From this wide range of concurrent but independent testimo- 

 nies we may accept it as practically demonstrated that the ice- 

 sheets disappeared from North America and Europe some six to 

 ten thousand years ago. But having thus found the value of one 

 term in our ratios of geologic time divisions, we may know them 

 all approximately by its substitution. The two inches assumed 

 to represent the postglacial portion of the Quaternary era may be 

 called eight thousand years ; then, according to the proportional 

 estimates by Davis, the Triassic period was probably two million 

 four hundred thousand years ago ; the time since the Carbonifer- 

 ous period, in the closing part of the Palaeozoic era, has been about 

 four or five million years; and since the middle of the Cam- 

 brian period, twice or perhaps four times as long. Continuing 

 this series still further back, the earliest Cambrian fossils may be 

 twenty or twenty-five million years old, and the beginning of 

 life on our earth was not improbably twice as long ago. 



Seeking to substitute our measure of postglacial time in 

 Dana's ratios, we are met by the difficulty of ascertaining first its 

 proportion to the preceding Glacial period, and then the ratio 

 which these two together bear to the Tertiary era. It would fill 

 a very large volume to rehearse all the diverse opinions current 

 among glacialists concerning the history of the Ice age, its won- 

 derful climatic vicissitudes, and the upward and downward move- 

 ments of the lands which are covered with the glacial drift. 

 Many eminent glacialists, as James Geikie, Wahnschaffe, Penck, 

 De Geer, Chamberlin, Salisbury, Shaler, McGee, and others, be- 

 lieve that the Ice age was complex, having two, three, or more 

 epochs of glaciation, divided by long interglacial epochs of mild 

 and temperate climate when the ice-sheets were entirely or mainly 

 melted away. Prof. Geikie claims five distinct glacial epochs, as 

 indicated by fossiliferous beds lying between deposits of till or 

 unstratified glacial drift, and by other evidences of great climatic 

 changes. In this country Mr. McGee recognizes at least three 

 glacial epochs. The astronomic theory of Croll attributes the 

 accumulation of ice-sheets to recurrent cycles which bring the 

 winters of each polar hemisphere of the earth alternately into 

 aphelion and perihelion each twenty-one thousand years during 

 the periods of maximum eccentricity of the earth's orbit. Its last 

 period of this kind was from about two hundred and forty thou- 

 sand to eighty thousand years ago, allowing room for seven or 

 eight such cycles and alternations of glacial and interglacial con- 

 ditions. The supposed evidence of interglacial epochs therefore 

 gave to this theory a wide credence ; but the recent determina- 



FOL. XLIV. 14 



