120 PRINCIPLES OF STRATIGRAPHY 



Walther (45:74-79), the greater part of Germany during the 

 Middle Zechstein period (Permic), extending from the Urals on 

 the east to the Armorican chain in France, Belgium, South Eng- 

 land and Ireland on the west, and bounded on the south by the 

 Bohemian mass, and the mountain chain (Vindelician) correspond- 

 ing to the present Danube plain. In this sea the great salt deposits 

 of North Germany were laid down from the complete evaporation 

 of the waters. Fault depressions accompanying earthquakes also 

 belong to this class. 



B. Constructional basins. 



1. Volcanic (pyrogenic) basins. These include a. crater 

 lakes, where water is enclosed by the built-up rim of the crater, and 

 b. depressions in a lava sheet produced at cooling and occupied by 

 water. Examples of the former are Crater Lake of Oregon (in 

 part a broken-down cone), the Soda Lakes of Nevada, and Lakes 

 Albano and Nemi, near Rome, and Agnano and Averno, near 

 Naples. Other lakes of this type occur in the Auv^gne district of 

 France, about Auckland, New Zealand, and in other volcanic 

 regions. (Davis-7 ijcS'o.)* Sometimes these lakes are hot, or con- 

 tain gases fatal to animal life. In the Caucasus a lake of this class 

 (Elbruz) lies at an elevation of 18,500 feet above the sea. Lakes in 

 depressions in lava flows are probably not common, and when they 

 are found they are mostly small. 



2. Chemical (hydrogcnic) basins. This class is limited to 

 small ponds, or basins built by tufas in hot spring and geyser 

 regions. The small tufa basins in mammoth Hot Springs of the 

 Yellowstone region are typical examples. Examples of large size 

 are so far unknown. (See Fig. 67, page 343.) 



3. Organic (biogenic) basins, a. Basins built by the growth 

 of vegetation in moist climates are not an uncommon feature in 

 the peat bog regions of northern countries, though the larger ones 

 of this class may be in part of the barrier type, the vegetable 

 growth merely damming back waters in a preexisting valley. A 

 more perfect example of a phytogenetic or vegetal basin is seen in 

 Lake Drummond in the Dismal Swamp of \'irginia and North 

 Carolina. (41.) This lake was originally 22 feet above sea-level 

 and 16 feet deep. It is completely surrounded and enclosed by a 

 mass of peat and vegetable material with a maximum thickness of 

 20 feet. At present the lake is only 6 feet deep and only about 16 



* Davis places them among obstruction lakes. 



