142 Trans. Acad. Sci. of St. Louis. 
digious size, with fragments so fresh to all appearances 
that they seem to come direct from some titanic rock- 
crusher. Hven the adobe soils of arid regions, when 
examined under the microscope, attest the strictly me- 
chanical origin of their finest materials. 
Under such climatic conditions basic or metallic masses 
of meteoric origin must, without appreciable change in 
chemical composition, remain indefinitely upon the sur- 
face of the ground. Meteoric minerals that are highly 
unstable under conditions of a moist climate do not in 
a dry climate readily assume more stable forms. 
Pebble Mosaics of the Desert. FEolation in the desert 
gives rise to certain characteristics of the soil not met 
with in moist lands. These features serve at once to ob- 
scure meteoric masses as effectually as does thick vege- 
tation. Singularly enough one of the most notable effects 
of the winds upon the dry soils is, by the removal of the 
finer materials, to impart a gravelly appearance to the 
ground. Most arid plains-mantles are mainly composed 
of fine loams. There are often sands; but as a whole 
there is really little gravel or coarse rock. 
That the desert loams have the appearance of gravels 
is due to the fact that the winds constantly carry away 
the loose fine materials. When the pebbles in the soil 
are more abundant than usual it is not uncommon to find 
areas acres in extent covered by a single layer of small 
angular stones as closely and as evenly set as in mosaics. 
Upon careful search it is probable that such pebble pave- 
ments will yield meteoritic material even more abund- 
antly than the bare rock-surface of the Canyon Diablo 
region. In the interests of astronomy and cosmic geology 
they are well worth systematic investigation. Applica- 
tion of the Indian trader’s methods and confinement of 
effort to a search for the ‘‘heavy stones’’ and ‘‘green 
stones’? might soon disclose means more certain than at 
present for distinguishing meteoric pebbles from the 
myriads of the smaller rock fragments with which they 
are mingled. 
