180 GEOLOGY. 



The last age is still in progress ; the others are in the 

 past. How long they were we know not; but that they 

 were each much longer than the age of man has thus far 

 been we have the most conclusive evidence to show. 



It is proper to notice here a division of time into five 

 periods which is common in works on Geology, and in 

 relation to which there are certain terms that are in con- 

 stant use by all geologists. 1. Azoic time. This has 

 been already explained. 2. Palaeozoic time. This was 

 a period in which the forms of life, as their remains 

 show, were very ancient that is, differing decidedly 

 from the present. This period includes the three ages of 

 mollusks, of fishes, and of coal-plants. Its name comes 

 from palaioSj old, and zoe. 3. Mesozoic time, the fossils 

 of which differ less than those of the Palaeozoic from the 

 living forms of the present, the term being derived from 

 mesos, middle, and zoe. The ages included in this period 

 are the middle ages of geological history, and cover 

 what is called the age of Reptiles, which is really, like all 

 the other grand divisions of geological time, a succession 

 of long ages. 4. Cainozoic (or, as it is sometimes called, 

 Cenozoic) time, from kainos^ recent, and zoe. This is 

 the age of Mammals. 5. The Present age. The terms 

 primary, secondary, and tertiary are used often as mean- 

 ing the same, respectively, as Palaeozoic, Mesozoic, and 

 Cainozoic. 



262. Boundaries of the Ages. No age has had a sharp- 

 ly-defined limit in relation to its life-record, but each 

 age has been to some extent mingled with other ages 

 in its rise with the age that preceded it, and in its de- 

 cline with the age that followed it. Geological history, 

 as Professor Dana remarks, is like human history in this 

 respect, ages in both cases being marked by peculiarities 

 which, indistinct at first and foreshadowed in a previous 

 age, at length come out prominently to view. When 

 they thus culminate it is easy to trace them back to their 

 rise, and follow their growth, and then their decline, as 



