HISTORICAL GEOLOGY 383 



Since in no one region are the strata of even a majority of the 

 systems of the earth represented, it is evident that one of the diffi- 

 culties of geology is to bring together the data and place them in 

 their true order so as to make a complete and accurate record. For 

 example, unconformities representing a loss of two or three systems 

 may occur in two sections, but when the two sections are compared 

 it may be found that they complement each other, that which is 

 lacking in the one being present in the other and vice versa. It is 

 evident that when such sections exist, a complete record of a portion 

 of geological time is available. 



The difficulties may be seen by a study of the rocks upon which 

 the city of Paris is situated. An examination of these strata shows 

 that, at least ten times in the past, this region was covered by the 

 sea and sediments accumulated on the sea floor, and as many times 

 the sea bottom was raised above the water and was subjected to 

 erosion. When the latter occurred, no sediments preserve the fossils 

 of the periods during which land existed, and it is only by studying 

 the fossils in strata of other regions that the whole history can be 

 read and the age of the strata which are present be determined. 



Divisions of Geological Time 



The broad outlines of the earth's history have been learned as a 

 result of such studies as those indicated above, and have been ar- 

 ranged in chronological order and separated into more or less clearly 

 marked divisions which correspond to the chapters of human history. 

 The divisions of time and corresponding divisions of the rocks have 

 been given the following terms : 



Time Scale Rock Scale 



Era Group 



Period System 



Epoch Series 



Age Stage 



An era consists of several -periods during which a group composed of 

 several systems of strata were accumulated. During an epoch 2l series 

 composed of one or more stages was laid down. When one speaks of 

 the Cambrian System, he means the succession of strata which were 

 laid down in the Cambrian Period; when he speaks of the Miocene 

 Series, he refers to strata deposited during the Miocene Epoch, i.e., 

 during a definite portion of the Tertiary Period. 



CLELAND GEOL. — 25 



