THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 5 1 



also be considered the effects of elevation or depression of 

 the interior of land masses upon the amount of detritus 

 carried down to the sea borders, there to be made into sedi- 

 ments. With all the errors of estimation there is, however, 

 a real value to the time-ratios of Dana, and to legitimate cor- 

 rections deduced from study of the same facts, which cannot 

 be denied ; the principle of time-ratios may be used as a 

 " working hypothesis" until something better is devised. 



Systems the Standard Units of Geological Chronology. In 

 the preparation of a universal time-scale for the history of 

 organisms, systems are the actual facts in nature which are 

 accepted everywhere as standard units of chronology. 



Geological Eras and Times and their Names. Whatever may 

 have been the actual length of time occupied in the making of 

 any one of them, or however much the estimates of the rela- 

 tive value of each may differ, it is certain that each system in 

 its particular region represents that particular part of the 

 geological time-scale during which the fauna and flora whose 

 remains it contains lived. 



This portion of time may be called the life era of the 

 organisms which make up the fossil fauna and flora of each 

 system. 



The names of these systems may then be applied directly 

 to the eras, and we thus have in the time-scale ten eras, 

 viz., the Cambrian era, the Ordovician era, the Silurian era, 

 the Devonian era, the Carboniferous era, the Triassic era, the 

 Jurassic era, the Cretaceous era, the Tertiary era and the 

 Quaternary era, including the present_time. In a time-scale 

 we know of the eras before the Cambrian only as precanibrian, 

 i.e., those that were earlier than the Cambrian. 



The first five of these eras are classed together as Paleozoic 

 time, the next three eras are called Mesozoic time, and the 

 Tertiary and Quaternary constitute Cenozoic time. 



Division of the Eras into Periods. Fossils have been col- 

 lected and studied with different degrees of precision for the 

 several eras and in different parts of the world, but, taking 

 the present stage of knowledge of fossil organisms, the paleon- 

 tologist is able to distinguish about twenty different successive 



