30 University of California Publications in Geology t VoL - 7 



a regolith which very effectually hides such indications of strata- 

 fication as are present, the only exposures showing any struc- 

 tural characters whatever over considerable areas being exposures 

 of pseudostrata. 



Origin of subordinate layers. — A more difficult matter to 

 explain is the very distinct layers and lamellae into which some 

 of the pseudostrata are divided, and which are illustrated in 

 plates 4 and 6. These are frequently parallel-sided, sharply 

 defined and physically separable, either by simply lifting the 

 upper from the lower or after a light blow from a hammer, and 

 often with siliceous coatings separating them. 



While not visible as such, it seems possible that the cement is 

 deposited with a more or less banded structure due to different 

 penetrations and different levels of depositions in different 

 seasons. In hardpans, where calcareous or ferruginous cement 

 is abundantly developed, a roughly horizontal banding may be 

 distinctly visible. Any agent, then, producing fracture in the 

 rock, — temperature changes, plant action, even the drying out 

 and shrinkage of the siliceous cement, — would tend to break it 

 along the banding surfaces. 



In a specimen collected, a distinct silica-coated, roughly 

 parallel-faced layer about three centimeters thick, is a crack run- 

 ning somewhat obliquely to the bounding surfaces, and traceable 

 for about fifteen centimeters on one side and six on the other. 

 It is very distinctly not due to pressure or to faulting or shear- 

 ing, and it is apparently not a bounding line between two layers 

 of deposition of cement. It may be due to shrinkage or tempera- 

 ture changes. The plane determined by it is partly occupied by 

 a coating of silica. It is shown in plate 6, figure 3. 



Some of the silica coatings are peculiarly marked by an 

 irregular network of minute furrows like numerous small worm 

 tracks. These are evidently the impression of fine rootlets, as 

 individuals can be traced and their gradual tapering and branch- 

 ing distinctly made out. Such thin mats of interlocking rootlets 

 are quite competent to extend and complete cracks that may have 

 been started by other agents. They are to be expected only in 

 an exposure or quite near the surface. 



The best separation planes were noted in the pseudostrata 



