THE TERTIARY PERIOD 291 



present; and on the Pacific Coast fully 30,000 feet, with a thick- 

 ness of 10,000 to 20,000 feet shown in many districts. According to 

 these figures it is seen that the thickness of the Tertiary system 

 is quite comparable to that of ordinary Paleozoic or Mesozoic 

 systems. 



Igneous Rocks. — In the above descriptions, attention has 

 been wholly given to a consideration of the sedimentary rocks 

 (including some tuffs and volcanic ash deposits), but the igneous 

 rocks of Tertiary age are also of very great extent and importance, 

 particularly in the northwestern part of the United States. This 

 igneous activity will be discussed below in connection with the 

 Tertiary physical history of North America. 



Physical History 



In the interest of more clearly presenting an outline of our 

 unusually detailed knowledge of the complicated physical history 

 of this comparatively recent (Tertiary) period, we shall depart 

 slightly from our ordinary method by considering first the relations 

 of land and water, basins of deposition and character of sediments 

 in different parts of the continent, etc., after which will follow a 

 discussion of the development of relief features in the east and west, 

 and mountain making and igneous activity in the west. 



Atlantic Coast. — During most of the time from Eocene to 

 Miocene inclusive, much of the Atlantic Coastal plain (including 

 Florida) was occupied by marine water (see Figs. 180, 181). Certain 

 unconformities, especially between the Eocene and Miocene, show 

 that there were some retrogressions and transgressions of the sea. 

 During Eocene time the newly added belt of Cretaceous deposits 

 lay along the shore, and the Eocene strata are known to have been 

 mostly derived from the Cretaceous and in part from the more 

 inland older formations. In general, it may be said that, to and 

 including the Miocene, there was a tendency to gradually push 

 the shore line farther eastward by the addition of strips of land. 



With the possible exception of the Lafayette, which is usually 

 regarded as of Pliocene age, the only marine strata of Pliocene age 

 comprise the comparatively thin Waccamaw formation on the 

 middle Atlantic Coast. 



The Lafayette deposits of North Carolina, according to Ste- 

 phenson and Johnson, "are present as surficial coverings (10 to 40 



