METEORITE CRATERS — SPENCER 



323 



explained as in figure 3?>, Avhere a ring anticline has been formed by 

 the percussion. The inner walls of the crater are always much 

 steeper than the outer walls. The rim and outer slopes are formed 

 of the fragmentary material shot out from the crater, and the 

 amount of this material and tlie size of the blocks will show a gradual 

 decrease with the distance from the center. Some of the fragmen- 



a. Meteorite Crater : fracturing and tiltinj of :;tvalCL 

 iy outward exphsiori 



d. Meteorite Crater : rinj anticline ly percussivn 



C Mine Crater at La Boisselle, Trance. iCjiS 



FiGDRB 3. — Diagrammatic sections of craters. 



tary material would also fall back into the crater and so cover up the 

 shattered rock at the bottom. The known meteorite craters are all 

 approximately circular in outline and their inner and outer slopes 

 are symmetrical about the center; that is, the craters are figures of 

 rotation about a vertical axis. This is as would be expected in the 

 case of an explosion crater; but in the case of a percussion crater 

 it would happen only when the projectile strikes the surface 

 normally. 



72774—35 22 



