28 University of Calif ornia Publications in Geology l. VoL - 7 



portion, representing usually from about the same to double the 

 thickness of the core layers, is of sand cemented by silica, and 

 more or less colored by iron. 



These forms probably originated from plant rootlets which, 

 during their decay, became loci for the silica deposition which 

 is rather pure where the rootlets themselves originally were. 

 The outer sandy coatings are bound to the axial portions by 

 silica cement deposited upon and in extension of the siliceous 

 nucleus. They often have the appearance of stalactites. Some 

 small sand pendants are shown in plate 6 ; their external forms 

 in part broken and not terminated, in figure 2, and longitudinal 

 and transverse sections, showing the silica core and central hole, 

 in figure 1. 



Explanation of phenomena. — In seeking for an explanation 

 of these phenomena the position and attitude of the pseudostrata 

 with respect to the surface of the ground is of fundamental 

 importance. As to position, the more indurated layers always 

 have their tops near the surface, rarely over ten or twelve feet 

 below it. In attitude they generally agree with the surface 

 slope, sometimes dipping at a slightly greater angle than the sur- 

 face. In the cases observed where they dip at a less angle than 

 the surface, and then usually outcrop, the surface slope has 

 been very recently modified by erosion, such as the undermin- 

 ing action of a stream or land sliding and the consequently 

 accelerated hill wash in its vicinity. 



The general appearance is often that of three layers or more; 

 when three they are the upper friable below the soil, the 

 indurated, and the lower friable. The appearance of more than 

 three is due to pseudolamination of the indurated layer. There 

 is, however, essentially one layer to account for — the main in- 

 durated pseudostratum. This appears to be due to a superficial 

 belt of cementation within the weathering zone, and dependent 

 on the aridity of the climate, the lower limit of cementation being 

 the lower limit of migration of the silica under normal conditions. 



The action is, therefore, analogous to the formation of hard- 

 pan in soils 3 from which it differs chiefly in the fact that it 

 occurs in the midst of a rock formation below the definite soil 



3 See Hilgard, E. W., Soils (New York, 1910), pp. 162 and 183. 



