188 



First, then, in trachyte or volcanic rocks, the lava-flows being for the 

 most part horizontal, the rock fractures vertically, and the falling away 

 of pieces produces bluffs which are generally very nearly vertical. More- 

 over, from the nature of the tlow, horizontal lines or bands are left run- 

 ning across the faces of all the blutits. This latter is very characteristic 

 of the formation. 



Second. At the bases of the bluffs debris slopes commence, and sweep 

 down generally in graceful curves to a greater or less distance. 



Third. These slopes are seldom very steep for any great distance ; 

 the great fall from the mountain-summits to the valleys being by way 

 of high bluffs and comparatively gentle debris slopes. In other words,, 

 the total fall is very irregularly distributed over the distance from the 

 top to the base of the mountain. 



Fourth. The junction-line of the bluffs and debris slopes is almost 

 always distinctly marked. 



On the other hand, in quarzite formations — 



First. Bluffs vertical, or nearly so, are very common ; except in very 

 rare cases, there are no marked horizontal lines. 



Second, The junction between the bluffs and debris slopes are never 

 so distinctly marked as in trachyte. 



Third. On account of the fact that this rock breaks off in large angu- 

 lar fragments, and also on account of its great hardness, it will lie 

 at a much steeper slope than the other rock. From the same causes the 

 loose rock does not take on those beautiful sweeping curves so comHaon 

 in volcanic rocks, but have a certain stiffness of line. 



Fourth. The solid rock, from its great hardness and the manner of its 

 crystallization, is often found in very steep, yet quite regular slopes, 

 without taking on the form of bluff. A noticeable instance of this is 

 the most easterly of the ten peaks mentioned above. The fall from 

 its summit to Vallecito Creek on the east is 3,000 feet, in less than a 

 mile horizontal, or a mean slope of nearly 32°; yet it is a plain slope 

 of solid rock, more or less irregular of course, but having no bluff 

 in all that distance. On the north side of the same peak there is a slope 

 of somewhere between 60° and 80° for not less than 2,000 feet, yet there 

 is no part of it bluff". 



Still another point is the fact that in the metamorphism of the orig- 

 inal sedimentary rocks into quartzite, the great natural convulsions 

 attending that process have distorted the strata terribly, so that, as in 

 this particular region, a number of peaks in a small area may each have 

 its strata dipping at a different angle from all the rest. The effect of 

 this on topography may be seen in the sketch from station 38. The 

 high peak next to the last one on the lelt shows in a marked maune 

 that the strata inclines to the right or soutliward. The high peak ne; 

 the middle of the sketch, being in the center of upheaval, has its strat 

 vertical, while those farther to the right incline to the north. Thi i 

 latter fact is not so well shown in this sketch, but from some otht . 

 points of view it appears very plainly. These facts show how the forr 3 

 of the peaks may vary in the same kind of rocks ; but, as there is little c i 

 none of this upsetting of the lava-flows, there must result a distinc 

 type of njouutain-form for each. The peculiar crystallization of the 

 quartzite has also a marked effect on the forms. 



In accordance with these facts, we find that quartzite mountains are 

 generally much more rugged, but lacking the relief given to those in 

 volcanic regions by the contrast of the bluff's with the <?e6m-slopes. 

 The boundary of the quartzite on the north follows closely the national 

 divide. 



