A Trip to Mar del Plata 197 



ing from off shore. We walked a short distance beyond 

 the bath-houses, and then began a minute and careful 

 examination of the surface of the barrancas, which gave 

 evidence of having been deeply worn and cut by the 

 waves during the past winter. The exposures of the 

 strata at this point represent the Upper and Middle 

 Pampean beds, as they have been called by Roth. The 

 Upper beds are light in color, having a yellowish 

 gray tint of varying shades of intensity; the Middle 

 beds are dark chocolate-brown. The Lower Pampean 

 beds, which are said to be red, are not exposed to view 

 at Mar del Plata, and Dr. Roth told me that to see them 

 it would be necessary to take a journey some twenty- 

 five miles to the south, for which unfortunately we did 

 not have the time. The material of which the Pampean 

 beds are composed is known by geologists as loess. 

 Loess is fine alluvium, which gives little evidence of 

 horizontal stratification, and which is therefore re- 

 garded by most authorities as having been to a very 

 considerable extent deposited by aerial agencies. The 

 fine dust originally brought down by the streams was 

 distributed by the winds, and the plants growing over 

 the region held the mass in place, and as the deposit 

 grew thicker, continued to hold it. Loess is everywhere 

 characterized by the presence of perforations more or 

 less perpendicular, these holes marking the place of 

 grass-roots and the stems of plants which long ago died 

 and vanished, leaving their molds in the fine material. 

 Into the openings thus left after a while limy and 

 silicious deposits were often carried by the water as the 

 rains percolated through the soil. At Mar del Plata 

 the loess is everywhere full of limy concretions, to 

 which the people of Argentina have given the name of 

 tosca. When these concretions occur in the soil of 



