300 G. D. LOUDERBACK STRUCTURE OF THE HUMBOLDT REGION 



leys. The range, as viewed from the Humboldt valley, appears crescent- 

 shaped, the concavity facing westward. The highest part is somewhat 

 north of the center, and the ridge line pitches to the north and south 

 until it passes underneath the valley in both directions. 



As seen from the Muttleberry road, the range seems to have a simple 

 anticlinal structure, the summit of the fold being close to the summit of 

 the road ; but northward from this summit the anticlinal axis passes 

 east of the crest, and southward it passes west of the crest. To the north 

 it is succeeded on the west by other folds, as described above, which, 

 while they do not form a large angle with the range trend, are distinctly 

 inclined to it. It follows that while the strike of the rocks is often par- 

 allel to the front of the range, yet it is frequently not so, and the result 

 is that the folds have their axes directed obliquely toward the valley and 

 are truncated by the plane of the primary valley ward slope. On account 

 of the varying curves of fold axes and range front, some folds on the west 

 side of the mountains * leave the range obliquely and are truncated on 

 the northern end of their axes, as the gypsum syncline above described ; 

 others are cut on the southern end of their axes. The highest obliquity 

 of strike to range trace was observed about 15 miles south of Lovelock, 

 where a resistant stratum of limestone forms a small promontory on a 

 Lahontan beach and strikes about north 82 degrees west, dipping south 

 at a high and variable angle. The range trace here runs about northeast 

 and southwest, and the limestone is obliquely intersected by the valley 

 line. 



In brief, the range shows a marked continuity of front, and the folds 

 of the bedrock series, while elongated in the same general direction as 

 the range, are distinctly discordant with it and vary in obliquity from 

 to 50 or 55 degrees. 



Igneous rocks. — Igneous rocks associated with the bedrock series are 

 not a prominent feature of the range; in fact, they hardly figure on the 

 west slope at all. However, considerable diorite was encountered along 

 the line of the detailed section, beginning near the summit, and exposed 

 at intervals to a considerable distance down the east side. It is intrusive 

 into the bedrock sedimentaries and underlies the superjacent series un- 

 conformably. Diabase boulders were also found in the Muttleberry 

 alluvial cone. Granite, which occurs in all the surrounding ranges, in- 

 cluding the Star Peak range, has not been found in the Humboldt Lake 

 range. 



Summary. — The bedrock complex consists principally of (Upper and 

 Middle) Triassic and Jurassic marine sediments, invaded at some points 

 by basic intrusives. It is universally folded, in places overturned and 



* There are practically none visible on the east slope, as will be seen later. 



