454 JAMES H. GARDNER 



segregate to like, subsequent to deposition of beds, is not sufficient to 

 account for many alluminous concretions in clays and shales. Often 

 the composition of inclosing sediments is closely similar to that of 

 the included concretion but is usually variable. 



The writer holds that many such concretions are contemporaneous 

 with the strata in which they are contained; that they have resulted 

 through adhesion of particles in overloaded water volumes disturbed 

 by currents. 



During the seasons of 1906 and 1907 the writer observed concre- 

 tions so formed under natural conditions in alluvial beds of Present 

 Age. This was in the desert region of the San Juan Basin, New 

 Mexico. Conditions were met with here such as are not common 

 to the present land areas. The Rio Chaco, some 40 miles above 

 its confluence with the Rio San Juan, may be taken as a type locahty. 

 Here the bed of the stream is made up of alternating layers of sand 

 and alluvial clay. Water flows along the bed only during the winter 

 and spring months or after extensive rains. The fall of the river is 

 very slight. During the flow, vast amounts of sand and clay, or mud, 

 are transported along by the sluggish stream. The water, disappear- 

 ing rapidly through evaporation and absorption in this arid region, 

 is forced to deposit its sediments along the way; first the heavy sand 

 grains or tiny pebbles, then the finer sand, then the coarse clayey 

 material, and finally the very fine silt, which is held in suspension, 

 becoming more and more concentrated as the water is soaked 

 up or is evaporated. Often this moving mixture is a mere viscid 

 fluid. After the water ceases to run and dries away, a thin coating 

 of clay is left over the surface of the stream bed. Resting in and 

 on this layer are often to be seen great numbers of round, concentric 

 clay concretions. The accompanying plate indicates the manner in 

 which they are collected into aggregations. These concretions are 

 sohd but may easily be broken with the hands. Some show nuclei 

 in the form of small pebbles or angular fragments but many of them 

 appear to be of similar material throughout with no recognizable 

 nuclei. Cross-sections revealed that some of them contain small 

 pebbles and sand grains in certain concentric shells of their makeup. 

 The majority average about i\ inches in diameter. 



The origin of these concretions is not difficult to explain. In 



