Keyes — Meteorites on the Painted Desert. 141 



the soil. A third portion falling upon desert tracts re- 

 mains exposed and is preserved unchanged for a much 

 longer time. But whether falling upon land or water 

 the stellar particles, on account of their high specific 

 gravity and their prevailingly metallic character, tend 

 sooner or later to sink beneath the lighter floating crust 

 of the lithosphere. 



In its ultimate analysis the meteoritic hypothesis is not 

 so radically distinct from Laplace's nebular hypothesis 

 as some of its advocates would have us believe. It is 

 not so entirely novel as it might at first glance appear. 

 As shown by G. H. Darwin 33 the meteoric swarm is dy- 

 namically analogous to a gas; and in reality the laws 

 of gases strictly apply. 



Peculiarities of Desert Rock Weathering. 



Insolation. The peculiarities of rock disintegration in 

 dry climates has an especial bearing upon meteoritical 

 augmentation in general, and in particular upon meteoric 

 phenomena displayed about Coon Butte. Without enter- 

 ing upon details emphasis may be laid upon the strictly 

 mechanical character of desert rock weathering. In arid 

 lands chemical rock decay is almost unknown. Destruc- 

 tion of rock-masses is accomplished mainly by spauling 

 due to great changes in diurnal temperatures at the sur- 

 face. To this distinctive geologic process the term inso- 

 lation is appropriately applied. 



As Russell 34 has pointed out, rock-decay appears to 

 be the direct result of normal climatic conditions ; in cold 

 or arid regions the rocks are scarcely at all decayed. The 

 surprisingly small extent of chemical decomposition 

 which rock-masses of the desert undergo is well shown 

 by the great talus slopes and other accumulations of col- 

 luvial deposits which form veritable rubble-heaps of pro- 



33 Phil. Trans. Royal Soc. London, 180:1-69. 1889. 



34 Bull. Geol. Soc. Amer. 1:134. 1890. 



