WILLIS] GENEEAL GEOLOGIC NOTES 25 



The horizon of the whitish-gray loess and of the pink pebbly loess 

 is characterized by great irregularity of deposits, and is thus dis- 

 tinguished from the higher horizon immediately overlying them, 

 which is occupied by a fawn-colored eolian earth most examples of 

 which are a structureless loess, but some of which exhibit columnar 

 structure. The fawn-colored loess is the highest of the formations 

 in the Barrancas del Norte and forms the upper third to half of the 

 bluff. It is continuous with the pink pebbly loess, which changes 

 gradually in color and loses the pebbly inclusions from below upward. 

 It is strongly contrasted in color with the whitish-gray and the plane 

 between them is clearly marked, but is flat and not eroded. Thus the 

 fawn-colored loess may be regarded as the upward continuaMon of 

 these two dissimilar deposits with which it corresponds in being of 

 wind origin. It merges upward into the black soil, which is derived 

 from it. 



In the foregoing descriptions of the original characters of the for- 

 mation in the Barrancas del Norte, the secondary feature, the occur- 

 rence of limestone masses, or tosca, has purposely been omitted, it 

 being regarded as a deposit from ground waters subsequent to the 

 deposition of the earth in which it occurs. 



There are two horizons in the Barrancas del Norte at which lime- 

 stone is strongly developed. One is near the base of the cliff, some- 

 what less than a meter below the upper surface of the dark-brown 

 sandy earth that forms the base of the exposed section. There is a 

 heavy horizontal plate of limey rock, in many places two-thirds of a 

 meter thick, and fairly constant in occurrence. According to the 

 writer's understanding of the process of tosca formation, it represents 

 the zone or horizon within which the ground water rises and falls, 

 while diffusing by capillary action and evaporating from the surface. 

 The conditions which are most favorable are those of semiaridity. 

 The density and continuity of the residual lime deposit constitute a 

 measure of the time during which the action continued at the hori- 

 zon, and as this formation is both dense and continuous it appears to 

 correspond to a notable episode. Thus, the lower plate or stratum 

 of tosca strengthens the evidence for an arid or semiarid epoch, fol- 

 lowing the deposition of the brown, sandy alluvium. 



The upper horizon of strongly developed tosca is at the base of 

 and in the fawn-colored loess that constitutes the upper third or 

 half of the barrancas. The limey rock occurs in irregular branching 

 masses that are longer vertically than horizontally and are more or 

 less separated from one another. The forms and occurrence are 

 adjusted to the structure of the loess, which is itself irregular and 

 occasions their irregularity. This horizon seems to indicate an 

 episode of climatic condition favorable to evaporation — semiaridity — 



