CHRONOLOGICAL SUCCESSION OF STRATIFIED ROCKS. 39 



logically investigated, such a chronological succession of the stratified 

 rocks has been established, but the order of succession is not neces- 

 sarily identical even in regions geographically close together. On 

 the contrary, as above pointed out, a comparison of the succession of 

 the stratified deposits in two regions widely remote from one another 

 in space will show that, though a general parallelism will exist, the 

 corresponding rock -groups in the two regions will not contain 

 absolutely identical fossils, and that certain rock-groups which are 

 present in one region are absent in the other. In no one region, 

 therefore, do we meet with an absolutely complete and continuous 

 succession of stratified rock-groups, nor could such ever have been 

 laid down except in a region which had been continuously beneath 

 the sea and constantly the seat of sedimentation since the beginning 

 of geological time. At all times of which we have geological rec- 

 ord, the earth's surface has, however, consisted partly of dry land 

 and partly of sea, and the terrestrial and marine areas have simply 

 undergone displacement and have been changed in position from 

 time to time. During each successive epoch, therefore, certain areas 

 have been the seat of sedimentation, while others have been dry land ; 

 but the dry land of one period may become the sea of the next, and 

 vice versa, and sedimentation is thus transferred in the course of ages 

 from one place to another. Hence when we meet with a stratified 

 deposit in one region (A) which has no representative in an adjoin- 

 ing region (B), we know that one or other of two things has occurred. 

 In the first place, B may have been dry land while A was beneath 

 the ocean. In that case, the missing deposit was never laid down in 

 B at all. Or, in the second place, both of the areas may have been 

 under the sea simultaneously, and the deposit in question may have 

 been originally laid down in both ; but A may have remained con- 

 tinuously under water, while B may have been elevated to form dry 

 land, undergoing in process of elevation sufficient denudation to 

 destroy the deposit in question. 



By a comparison of many different areas, geologists have been en- 

 abled to frame a genera/ order of succession of the stratified rocks, 

 which, though based originally upon the facts observed in Europe, is 

 nevertheless, in its main outlines, applicable to other and widely 

 distant regions. This general succession is diagrammatically shown 

 in the annexed ideal section of the crust of the earth. The most 

 ancient of all the stratified rocks are more or less intensely crystalline 

 in character, and no undoubted fossils have hitherto been detected 

 in them. They are grouped together under the general name of the 

 Archcean rocks, and comprise several rock-systems, of which the best 

 established is the Laurentian series of North America and of Europe. 

 All the rock-groups above the Archaean are more or less richly 

 fossiliferous, and are divided into three main " groups," each com- 



