324 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 3 



The most likely place to search for meteoric iron would be on the 

 rim and outer slopes, where much will be buried, perhaps to be 

 exposed later by weathering processes. Meteoric iron readily oxi- 

 dizes and breaks up. Buried fragments will give rise to the forma- 

 tion of more or less compact iron shale and shale balls, and these may 

 be expected to be more persistent than the iron itself. Silica glass 

 can be formed only when the rocks surrounding the crater are highly 

 siliceous, such as quartz sand or sandstone. 



NATURAL AND ARTIFICIAL ANALOGIES 



Basin-shaped depressions, often occupied by lakes, are common 

 enough on the earth's surface, and they may be formed in several 

 different ways. Volcanic craters and especiall}'^ caldera (formed 

 when the upper portion of a volcano is blown away by a violent 

 explosion, or by subsidence of the cone) may be very similar in 

 form to meteorite craters, but being composed of volcanic materials 

 they are quite distinct. But in the case of explosion craters (or 

 " embryonic volcanoes ") an explosion of steam and volcanic gases 

 may give rise to a crater consisting entirely of fragments of sediment- 

 ary rocks without the outpouring of any lava. Examples of these are 

 the " maars " of the Eifel. The Pretoria salt-pan ^^ (3,400 feet across 

 and 400 feet deep), 25 miles north-northwest of Pretoria, and Lonar 

 Lake on the basalt plateau of the Deccan, have been thought to be 

 such explosion craters; but the former presents more points of 

 resemblance to a meteorite crater than does, for example, Lake 

 Bosumtwi. Explosions of gas may also take place in oil-bearing 

 regions, sometimes with the formation of mud volcanoes. 



The craters on the moon are usually thought to be of volcanic 

 origin, but the suggestion has also been made that they were formed 

 by the fall of meteorites. Their large size is perhaps related to 

 the smaller force of gravitation. If the meteoritic theory is here 

 true, we can only hope that the earth is not approaching the same 

 stage. 



Craters may also be formed on the earth's surface by the solu- 

 tion of beds of limestone, or of pockets of rock salt or gypsum. 



As noted above, the craters of Estonia and of the Campo del Cielo 

 have been considered by some authors to be earthworks made by man. 

 This suggests that some other supposed earthworks may possibly be 

 really meteorite craters. Many dew ponds are clearly artificial, but 

 some supposed to be of Neolithic age may possibly have been formed 

 by the fall of meteorites. As examples of large holes made artificially 

 in the earth's surface mention may be made of the diamond mines 



'■'^ Wagner, Percy A., The Tretoria salt-pan, a soda caldera. Mem. Geol. Survey South 

 Africa, no. 20, 136 pp., 19 pis., 1922. 



