84 REPORT UNITED STATES GEOLOGICAL SURVEY. 



finely subdivided; a fact which gives the rocks a pleasing appearance. 

 Erosion produces some exceedingly picturesque effects. Frequently one 

 may ride in a narrow caHon, the walls of which are composed of these 

 hard shales and are 1,000 feet high. Erosion has affected these walls 

 in such a manner that the suspicion of having been utilized as models 

 for architectural ornamentation is almost involuntary. Beautiful carv- 

 ing, dependent upon the superior hardness of certain strata, has resulted 

 in the jjroduction of ornamented chancels and long-continued benches, 

 which stud the vertical enclosing cliffs. Long excavations have been 

 worn into the sides of the walls by the action of flowing water, and not 

 unfrequently can the traveller ride in shady coolness for some distance 

 ander a protecting natural awning. On the summits of ridges and 

 hills, from where the slopes fall off very steeply or are vertical, the pe- 

 culiar effect of atmospheric erosion can be studied. Prominent points, 

 the edges of bluffs, or precipitous walls show gracefully-executed carving. 



It can be compared with the tufaceous deposits near some mineral 

 springs. The soft thin laminae have been worn away, grooves de- 

 note their places, while the innumerable harder ones project like scat- 

 tered leaves of a book from the wall. All of the forms observed are 

 rounded, so that the total effect produced is that of a deposit formed by 

 overflowing springs. Oxydation of a slight percentage of iron pro- 

 duces changes of color varying from the palest yellow to pink, pale 

 orange, and a light brown, while the remaining strata and lamince are 

 white and grey. Looking at such a wall from a distance, all these minor 

 details are, of course, lost, and only the main features will be recog- 

 nized. This character is especially applicable to the highest members 

 of the shale series. Although it certainly exists in the lower portion, 

 it is not so prominent, owing to the occasional admixture and intersper- 

 sion of arenaceous strata. One of the best localities within the <listrict 

 for the study of these features is along the steep western edge of the 

 dividing ridge between the Pi-ce-ance and Douglas Creeks. In the 

 lower canon of Evacuation Creek the higher members of the shales 

 reach down to the level of the stream, and there exhibit in the most 

 beautiful manner the effects of erosion as produced by flowing water. 



Near the base of these shales we frequently find heavy beds of sand- 

 stone, separated from each other by interstrata of shale. The former 

 are generally white or light yellow, while the latter are of a yellow to 

 yellow-brown color. So far as 1 could observe, the sandstones grow 

 thicker as we proceed westward; it may be, however, that at the 

 points of my observation we had merely a local thickening of the beds. 



A large portion of the springs issuing from the shales are alkaline, 

 and almost all the water which flows over them, for evea a short dis- 

 tance, partakes of the same character. This is due to decomposition of 

 the constituents of both shales and marls (the latter in the lower por- 

 tion), which form new compounds, soluble in water. Numerous "alkali- 

 flats" were observed, where carbonates of soda and potash colored the 

 surface as freshly-fallen snow would. Our examinations were made 

 there during the driest season of the year, the most favorable to evapo- 

 ration, and, therefore, concentration of the alkalies in solution. Several 

 springs were found emitting carbonic acid gas and sulphuretted hydro- 

 gen. They contained ferric compounds and a very liberal supply of 

 alkali in solution. All of them were cold. This feature — alkali-water — 

 is certainly a drawback to many otherwise pleasant regions. 



As a total thickness for these shales, including their lower, arenaceous 

 members, about 2,400 feet may be given. This was obtained mainly 

 from the southern bold escarpment of the plateau, and corroborated by 



