PRELIMINARY OUTLINE. 



15 



of the sea, frequently contain the shells and skeletons of animals, and 

 sometimes the impressions of plants. Most of the relics of life found 

 in the stratified rocks belonged to animals or plants which lived in salt 

 water. Because of their structure, their composition, their distinctive 

 markings, and the remains of hfe which they contain, it is confidently 

 inferred that most of the stratified rocks which lie beneath the mantle 

 rock of the land were originally laid down in beds beneath the sea, and 

 that the familiar processes of the present time furnish the key to their 

 history. 



Fig. 1. — Beds of (Cambrian) sandstone, a, are conformable with one another, but 

 unconformable on beds of (Huronian) quartzite, h. Near Ableman, Wis. 



Conformability. — When the stratified rocks exposed by the re- 

 moval of the mantle rock are examined, the successive beds are some- 

 times found to lie on one another in regular succession, showing that 

 they were laid down one after another, without change in the attitude 

 of the surface on which they were deposited. Such rocks are conform- 

 able (the beds of series a, Fig. 1). In other cases it would be seen 

 that certain beds overlie the worn surfaces of lower beds, the layers 

 of which may have a different angle of inclination (series a, Fig. 1, 

 is unconformable on series h). Such relations show that the lower 

 series of beds was disturbed and eroded before the overlying beds 

 were deposited on them. Such series of rocks are unconformable. 



Relative ages. — The structure and relations of rocks lead to infer- 

 ences as to their relative ages. In the case of stratified rocks it is 

 obvious that overlying beds were deposited later than those below, 

 and where there is unconformity it is evident that an interval of time 

 elapsed between the deposition of the unconformable series. Another 

 and in some respects more important means of telling their order of 



