90 IOWA ACADEMY OF SCIENCES. 



the leaching progresses farther and farther down from the sur- 

 face, so that the width of the leached zone comes to be an index 

 to the age of the soil. It is obvious that the amount of leach- 

 ing is really proportional, not directly to the time, but to the 

 amount and strength of solution draining through the soil. An 

 exposed point may be subjected to a greater amount of solution 

 running through it. Conceivably also, the strength of the 

 solution might vary from point to point and from time to time. 

 A leached zone, then, to have value in this connection must be 

 shown to be general, and these local factors must be eliminated. 

 A widespread and well marked zone with the strength of the 

 acid reaction uniformly proportional to the distance below the 

 presumed horizon can, however, hardly be explained, except 

 as a true index of time. It is believed also, that at least in an 

 approximate degree, the amount of leaching shown by two sur- 

 faces gives a reliable means for comparing their ages. It may 

 be noted in passing that to get accurate results hot acid should 

 of course be used in testing till derived from dolomitic regions. 



Ferretto Horizons. — To those who live in the southern jDortion 

 of the state where the Kansan drift is exposed beneath the 

 loess, no phenomenon is more common than the reddish-brown 

 horizon marking the upper limit of the drift This old, red 

 soil, for such it is, is of the type known to the Italian geologist 

 as ferretto, and the name seems fitting and is useful. The 

 ferretto zone is manifestly due to the high state of oxidation of 

 the iron. The red-brown color shades off through orange and 

 yellow into the blue of the lower portion of the till, the change 

 being gradual, and the yellow clay being usually ten to thirty 

 feet thick. 



The reddish zone is narrower, and while its lower limit is 

 naturally but poorly defined, the ferreto zone proper is usually 

 but two to three feet in thickness. The progressive increase 

 in the oxidation of the iron toward the surface is accompanied 

 by a similar increase in general oxidation, and increasing rotten- 

 ness of the bowlders and pebbles. There are exceptions and 

 fresh bowlders occur well to the top, and even on the surface 

 of the drift, while rotted cobbles are found to the bottom. Such, 

 however, is not the rule. In the formation of ferretto and in 

 the broader work of general oxidation and decay of pebbles 

 local causes favoring or hindering the action come into play, 

 and it is the relations of the phenomena to an old general sur- 

 face that cause its significance. The local variations are 



