56 E. HUNTINGTON- J. IV. GOI^DTHWAIT 



layers of alternating hard and soft strata are truncated by a level 

 lava-covered surface, although at present hard formations lying in 

 the same attitude form marked ridges. Surfaces of all these vari- 

 ous types have been covered by lava flows, and have been exposed 

 to view by a recent fault or series of faults, and the renewed ero- 

 sion consequent on uplift. They all indicate that between the 

 earlier and later times of faulting there was a long period of 

 erosion, at the end of which the region had been reduced to rela- 

 tively low relief. 



d^ Mature topography of the Co lob plateau. — Two miles north 

 of Kanarra coarse Pleistocene gravels rest on the rather level 

 surface formed by the dissection of the inverted strata between 

 the old and new faults. On the gravel lies lava which seems to 

 have come from the high plateau of Colob, east of both faults. 

 The lava probably did not cross far west of the old fault, because 

 it soon encountered the down-thrown Aubrey limestone which 

 stood relatively high on account of its hardness. The recent 

 faulting depressed the limestone area and allowed it to be cov- 

 ered with deep alluvium. This displacement also gave rise to a 

 renewal of erosion by which the lava on the edge of the up-thrown 

 block has been dissected into mesas and buttes, although back 

 from the borders on the main mass of the maturely dissected 

 plateau the revived activity of erosion has not as yet penetrated. 

 The lava there lies on a surface of just the same sort as that which 

 still characterizes the neighboring parts of the plateau which are 

 not protected by a basalt covering. So it seems probable that 

 the topography of the central mass of the plateau, even where it 

 has not been protected by lava, is almost the same as that which 

 prevailed previous to the basalt flows and the second faulting. 

 The rounded mountainous hills, rising to a height of two or three 

 thousand feet, the graded slopes, well-established drainage, and 

 broad valleys, are strongly in contrast to the precipitous slopes 

 and new aspect of the peripheral regions, and seem to represent 

 the conditions previous to the second uplift. 



_ e) Differe?itial recession of cliff s o?i the tivo sides of the old faidt.- — 

 Still another line of evidence shows that there have been two 

 periods of faulting separated b}' a considerable interval of ero- 



