142 Trails. 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. Even 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. Eolation 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. 



