MINERALOGICAL GEOLOGY 
415 
tern-lake beds are in evidence; no inconsiderable number of the 
lithologic units are characteristically epeirotic or continental in 
nature; there are many lava flows included.; there are rock-salt, 
gypsum, chemically formed limestones, and volcanic ash beds in 
rapid recurrence; there are apparently no playa deposits, or any¬ 
thing that approaches them. Judged by this strange changeable¬ 
ness of strata the region was sometimes under the sea, sometimes 
out of the water, sometimes marshy, sometimes dry land with the 
water-table only a few feet beneath the surface of the ground. 
Whether desert conditions of today ever obtained is conjectural, 
with the probabilities against them. , 
Under the last mentioned conditions especially a peculiar and 
perhaps unique situation is often inaugurated in semi-arid tracts 
and certain spots in the desert. In the dry, thin air excessive 
evaporation draws the mineral-ladened ground-waters to the sur¬ 
face, where, the water passing off, is left a deposit of complex 
salts. The more soluble salts are speedily disposed of by the in¬ 
frequent rains. But the lime and less soluble salts often remain 
behind to form a hard surface layer which Mexicans aptly desig¬ 
nate “caliche.” The caliche stratum is a few inches to 30-feet or 
more in thickness. It is essentially an earthy lime-rock, very por¬ 
ous, light, fluffy, and illy bedded. Its notable peculiarity is a 
• characteristic nodular aspect, the individual ‘ nodules varying in 
bulk from the size of a man’s fist to that of his head, and even 
larger, the whole buried in a white limy matrix. Such calcareous 
deposits form into irregular beds. When excavated by the steam- 
shovel in railroad grading operations the masses fall readily to 
pieces and the loaded cars present the appearance of gravel or 
boulder burdens. 
A most noteworthy feature about this nodular caliche is its gen¬ 
eral structural or textural resemblance to the colemanite beds. 
Solfataric waters passing over such a surface that caliche presents 
seemingly could easily alter it more or less completely. With 
numerous chloridic, sulphatic and other salts also present and the 
waters carrying boracic, sulphuric and other acids a complex set 
of reactions must ensue such as rarely or never obtain in the 
chemical laboratory. 
Possibble derivation of colemanite beds from caliche obviates 
every difflculty which attaches to the normal limestone hypothesis. 
