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University of California Publications in Geology [Vol. 11 



In this pinching and swelling, the thickening of one bed is quickly 

 compensated by the thinning of adjacent beds. In this way, beds 

 which show noticeable thickening and thinning may lie between beds 

 which are evenly bounded and uniform in thickness. The pinching 

 and swelling is not due to thickening and thinning of the beds in 

 folding. This is, shown by the fact that the thick parts and the thin 

 parts have, as a rule, no regular distribution, and show no relation to 

 the axes of folding. Pinching and swelling occurs in beds which are 

 not greatly contorted. There is some thickening and thinning in 

 folds, but the greater part of the variation in thickness does not have 

 this origin. 



Another peculiarity of bedding is the occurrence of rounded 

 knobs on the surface of the chert beds (center of plate 27a). The 

 chert may have a fairly uniform thickness over an area of a few 

 square feet, but at one point there is a piling up of the chert sub- 

 stance in a small mound or knob. The nature of the chert in the 

 knob is exactly like that of the rest of the bed, so that the cause of the 

 swelling is not due to differences in material. In this respect these 

 knobs are distinct from certain concretionary masses found in the 

 cherts. The knob is limited to a single bed, and its irregularity is 

 quickly compensated by variations in the beds above. These knobs 

 are very common in the well bedded cherts. Their dimensions are 

 generally not very large ; the whole knob may have an area much less 

 than a square foot, and the increase in thickness may be as much as 

 two inches. Figure 4 shows a case where a thin shale parting dis- 

 appears when it runs into a knob of this sort. This figure illustrates 

 a case where the knob affects both the top and bottom of the bed. 

 Often only one side is affected. 



Fig. 4. Termination of shale parting at a place of unusual thickness 

 in a chert bed. 



The cherts sometimes occur in short lenses and in nodules, some 

 of which are unusually thick (figures 5-7). These occasionally are 

 two feet in horizontal dimensions and as much as six inches thick. 

 The adjacent beds curve around such large masses, but by variations 



