PALEOBOTANY 197 



few inches near the margin to some 300 feet near the crater. 

 In 1883 the eruption of Krakatoa, in the Straits of Sunda, 

 killed practically all the plants and animals on an island 

 of five square miles in area, and on neighboring islands ; 

 a part of the island was completely blown away, leaving 

 only deep water. So recently as 1912 the eruption of 

 Katmai, in Alaska, spread a layer of ash nearly a foot deep 

 over the entire surface of Kodiak Island, one hundred 

 miles from the volcano, and killed all the herbaceous vege- 

 tation, leaving only trees and bushes. It is almost certain 

 that many species of plants and animals have become ex- 

 tinct by such agencies. Not only the lava, but poisonous 

 gases that frill the air during volcanic eruptions, may 

 prove fatal to plant and animal life. 



7. Encroachment of salt water in coastal regions, 

 caused by changes in the level of the land, resulting in the 

 killing of fresh- water vegetation. According to Fernald, 

 one of the sundews, Drosera filiformis, is known to occur 

 in only two regions, namely along the Gulf coast from Flor- 

 ida to Mississippi, and along the Atlantic coast from Mary- 

 land to Massachusetts (Fig. 85). Its extinction in the 

 intervening region is explained by the subsidence and 

 drowning of a former high continental shelf, along which 

 this and other species migrated northward during the 

 late Tertiary. If a similar subsidence should occur in the 

 two limited regions where the species is now found it 

 would become extinct unless, by some combination of 

 circumstances, it could migrate and become established 

 in new localities. It is not unlikely that species have 

 often been exterminated in this way. 



8. Encroachment of Fresh Water over Land Areas. 

 Previous to about the year 1900, the Salton basin, in 



