24 LAKES OF NORTH AMERICA. 



has already been referred to in connection with the formation of basins 

 through the action of eolian agencies. 



Lava streams frequently cool on the surface while the liquid rock below 

 is still flowing. In such instances, when the crust is sufficiently strong 

 to sustain itself, the molten lava beneath flows out, leaving caverns. 

 Openings of this nature may become water-filled and form subterranean 

 lakes, or their roofs may fall in, leaving depressions open to the sky. 

 Lakes and ponds occupying such depressions are thought to exist on the 

 vast lava sheets of Oregon, Washington and Idaho, but clear, simple 

 examples of the type are not at hand. 



On a small lava flow on an island in JNIono lake, California, there are 

 depressions occupied in part by Avater, which are due to a general sub- 

 sidence of the surface on account of the outflow of molten rock below and 

 the crumpling of the crust into concentric, crescent-shaped ridges. 



Another mode in which volcanic agencies may produce depressions is 

 by subsidence of the surface about volcanoes, due to the removal of lava 

 from subterranean reservoirs, but no instances where this has certainly 

 occurred have yet been observed in this country. 



Basins due to the impact of meteors. — The study of the origin of 

 the crater-like forms on the surface of the moon recently made by Gilbert,^ 

 was suggested by the hypothesis that depressions on the earth's surface 

 might result from the impact of meteoric bodies. This suggestion has 

 already been referred to in describing Coon butte, and is one of great 

 interest. Up to the present time, however, no basins on the earth's sur- 

 face are known which can be ascribed to this agency. 



If the earth was formed by the coming together of a large number of 

 previously independent meteoric bodies, as is thought to have been the 

 case by Lockyer, all evidence of such an occurrence in the relief of its 

 surface is wanting. Small meteors are known to reach the earth every 

 day, and a number have been discovered weighing many tons, but such 

 an event as the earth coming in contact witli a planetary mass a mile or 

 several miles in diameter, as seems to have happened in the case of the 

 moon, is not onl}^ unrecorded in history, but, as just stated, there is no 

 evidence in the surface features of the earth to show that such an 

 event has happened in recent geological time. If the earth once had 

 a pitted surface, like the moon, and was scarred by vast crater-like 



1 " The Moon's Face," Philosophical Society of Washington, Bull. vol. 12, 1893, pp. 

 241-292. 



