HYPOTHESES AS TO ORIGIN - 219 



have been formed by the rupture of previously hardened lava. The 

 specimens collected for examination were found to have the same mega- 

 scopic and microscopic appearance as cinders from volcanic cones and 

 gave no evidence for or against the hjrpothesis suggested. 



The presence of the angular blocks of basalt in the rims can be satis- 

 factorily accounted for only on the supposition that they came from the 

 underlying basalt sheet. No sand mounds were found elsewhere on La 

 Mesa which are at all comparable in elevation with the crater rims; 

 hence there is nothing to suggest that the rims are in any sense rem- 

 nants left by a general degradation of the plain. There is, furthermore, 

 good evidence in the general character of the plain that its present sur- 

 face is essentially the same as that originally formed by the deposition 

 of the sands. 



Apparently the only objection to the explosion hypothesis is the prob- 

 able discrepancy in volume between the craters and the rims. The 

 volume of the rims is apparently less than that of the depressions from 

 which the material is presumably derived, but a measure of the differ- 

 ence can be given only after much more detailed surveys have been made. 

 As previously stated, however, the action of the wind is adequate to ac- 

 count for a considerable diminution in the size of the rims. The im- 

 portance of the wind as a transporting agent in this region is shown by 

 the great quantities of drifting sand and by the numerous areas of gravel 

 due to the surface concentration of coarser material that are found in 

 many places on the surface of La Mesa. It is not improbable that the 

 crater rims may originally have been volumetrically equal to the depres- 

 sions and have been diminished to their present volume wholly through 

 wind action. A certain amount of diminution by this means is proved 

 by the fact that the rims are covered more or less completely with frag- 

 ments of lava, small pebbles, and sand too coarse to be readily moved by 

 winds. 



It is furthermore probable — and for this suggestion I am indebted to 

 Dr Whitman Cross — that explosions having force sufficient to blow out 

 such large craters would throw much of the material too far away to be 

 included in a recognizable rim. 



The other two hypotheses may be possible, but can scarcely be consid- 

 ered probable. The presence of the craters in a volcanic region within a 

 few miles of vents from which considerable quantities of lava have been 

 extruded is in some measure favorable to the third h3rpothesis. The fact 

 also that the volume of material in the crater rims is apparently less than 

 that of the depressions might be interpreted as favorable to this hypoth- 

 esis, were it not for the more probable explanation just given. 



XVII — Boll. Geol. Soc. Am., Vol. 18, 1906 



