474 GEOLOGICAL SURVEY OF THE TERRITORIES. 



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 flow, horizontal lines or bands are left run- 

 ning across the faces of all the bluffs. 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 quartzite formations — 



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

 raie cases there are no marked horizontal lines. 



Second. The junction between the bluffs and debris slopes is 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 common 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 irregular 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 at 

 an angle of 00° to 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 origi- 

 nal sedimentary rocks into quartzite, the great natural convulsions at- 

 tending 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 left shows in a marked manner 

 that the strata incline to the right, or southward. The high peak near 

 the middle of the sketch, being in the center of upheaval, has vertical 

 strata, while those farther to the right incline to the north. This lat- 

 ter fact is not so well shown in this sketch, but from some other points 

 of view it appears very plainly. These facts show how the form of the 

 peaks may differ in the same kind of rocks ; but as there is little or none 

 of this upsetting of the lava-flows, there must result a distinct type of 

 mountain-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 bluffs with the debris-slopes. 

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

 divide. 



