PERMIAN AGE AND CLOSE OP THE PAh^OZOIC. 175 



an age or period in the history of the earth, as these 

 terms are understood by geologists ? In most geolo- 

 gical books terms referring to time are employed very 

 loosely. Period, epoch, age, system, series, formation, 

 and similar terms, are used or abused in a manner 

 which only the indefiniteness of our conceptions can 

 excuse. 



A great American geologist* has made an attempt 

 to remedy this by attaching definite values to such 

 words as those above mentioned. In his system the 

 greater divisions of the history were ^^ Times : " thus 

 the Eozoic was a time and the PalaBOzoic was a time. 

 The larger divisions of the times are ^'Ages:^^ thus 

 the Lower and Upper Silurian, the Devonian, and 

 the Carboniferous are ages, which are equivalent in 

 the main to what English geologists call Systems of 

 Formations. Ages, again, may be divided into 

 ''Periods:^' thus, in the Upper Silurian, the Ludlow 

 of England, or Lower Helderberg of America, would 

 constitute a period. These periods may again be 

 divided into " Epochs," which are equivalent to what 

 English geologists call Formations, a term referring 

 not directly to the time elapsed, but to the work done 

 in it. Now this mode of regarding geological time 

 introduces many thoughts as to the nature of our 

 chronology and matters relating to it. A '^ time '* in 

 geology is an extremely long time, and the PalaGOzoic 

 was perhaps the longest of the whole. By the close 

 of the Palaeozoic nine-tenths of all the rocks we know 

 • Dana. 



