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*? 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®* 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 icol- 
luvial deposits which form veritable rubble-heaps of pro- 
* Phil, Trans. Royal Soc. London, 180:1-69. 1889. 
* Bull. Geol. Soc. Amer. 1:134. 1890. 
