MESOZOIC AND CENOZOIC GEOGRAPHY 467 
The alteration which has occurred among the rocks of these 
mountains, has caused the accumulation of very important deposits 
of minerals. Associated with the slates are veins of quartz, in 
which gold occurs in considerable abundance. These gold-bear- 
ing quartz veins are now being worked; and gravels, accumulated 
in stream beds as a result of the disintegration of the slate and 
quartz, have furnished large stores of this precious metal. It is 
not certain whether the gold was originally disseminated through 
the clay beds, and then gathered into veins, or whether it has 
come from some lower source in the earth, and has filled the 
veins. 
Cretaceous Geography. — The prevailing land con- 
dition of the Juratrias was gradually succeeded, dur- 
ing the Cretaceous period, by the invasion of a part of 
the country by the sea (Fig. 262). The Atlantic Ocean 
extended over the eastern part of the present sea- 
board states south of New England. Nearly the 
whole of these states, from New Jersey to Georgia, 
were covered by the sea; Florida and the greater 
part of Alabama and Mississippi were also submerged. 
From the Gulf shore northward, a branch of the sea 
extended apparently as far as the Arctic circle, and 
the states between the Mississippi River and the Rocky 
Mountains were covered, while the sea invaded a con- 
siderable portion of the Cordilleran region. 
In some places this sea was very deep, and conditions 
existed like those now favorable to the accumulation 
of Globigerina ooze in the deep sea. Chalk beds were 
then formed in Texas, Iowa, and other places. 
