i *rr*r^^r-' M ■ 



THE VICINITY OF LA PORTE AND GIBSON V I LLE. 



445 



it changes its direction would lie farther to the west than the serpentine that I saw on the trail 

 from Downieville to Excelsior, on the trail from Deadwood to Poker Plat, and near Whiskey 



Diggings or Newark. 



The highest points are capped with a basaltic or an andesitic lava, and upon the crests of the 

 ridges between the canons the lava extends in almost unbroken streams nearly to the southern 

 and southwestern limits of the district. The mountain ridges at the head of the canons are much 

 more irregular in outline than are the ridges between the forks of the Yuba Eiver in Nevada and 

 Sierra Counties, which have been described in the previous pages. There is very little of that 

 smoothness and evenness of grade which is so marked a feature of the more southern country. 

 Sharp peaks alternate with deep sags, and their sides are deeply furrowed with secondary canons 

 and gulches. There are also many evidences of the frequency of land slides. I could not find 

 time to climb any of the higher peaks, nor to make a much needed examination of the country at 

 the head of Canon Creek. The principal end in view in such an examination would be to ascer- 

 tain the relations of the volcanic material to the bed-rock. I was told by persons well acquainted 

 in the region that the bed of Canon Creek is crossed by volcanic rock near Poker Plat. In the 

 short time at my disposal I was unable to satisfy myself in regard to this matter. The serpentine 

 belt appears, it is true, to abut directly against volcanic rock ; at least I saw none of the character- 

 istic slate at the point where I crossed the creek, but it is possible that the appearance is due to 

 the effects of land slides, and that the volcanic rock seen was not in place. I must leave this 



question unsettled. 



In the descriptions which follow it will be convenient to take up first the more northwesterly 

 portions of the district, beginning with the vicinity of La Porte and Gibsonville, and then to take 

 in order the successive ridges which lie to the southeast. 



A. The Vicinity of La Porte and Gibsonville, West of Slate Creek. 



I reached La Porte on the evening of the 8th of September by stage from Oroville, by way 

 of Porbestown and Brownsville. The bed-rock observed by the roadside after leaving Browns- 

 ville was nearly all slate, !Nb volcanic rock was seen. The highest point on the road is about a 

 mile to the west or southwest of La Porte, on the crest of the ridge which separates the waters of 

 Slate Creek from those of East Creek and other affluents of the South Pork of the Feather Eiver. 

 The town of La Porte lies on the southeastern slope of this ridge, upon Eabbit Creek, by which 

 name the town also was formerly known. The altitude of the second story of the Union Hotel at 



La Porte I made to be 4,993 feet. 



The map (Plate E), which is one of those for which I am largely indebted to the kindness of 

 Mr. C. W. Hendel, shows the principal geographical features of the region, and the relative posi- 

 tions of bed-rock, volcanic tufa, and exposed gravel. My explorations did not extend farther 

 south than the trail from Barnard's to Poverty Hill. Near this point the old channel crossed to 

 (he opposite side of Slate Creek, and its continuation in that direction will be one of the subjects 

 taken up in the next section of this report. It will furthermore be convenient to consider the 

 vicinity of La Porte as a sub-district by itself, not much being known about the half-dozen miles 

 which intervene between La Porte and Gibsonville. 



(I.) A little more than a mile to the northwest of La Porte may be seen upon the ridge the 

 lower extremity of the volcanic capping, which extends from that point, without interruption, i 



in 



a northeasterly direction to Pilot Peak. The La Porte Bald Mountain is a prominent object on 

 the ridge, and lies a little to the east of north from the town. I determined the altitude of the 

 sag to the west of Bald Mountain, at a point near the line of the proposed ditch tunnel, to be 5,500 

 feet, and estimated the top of the mountain to bo two hundred and fifty feet higher. The altitude 

 of the base of the lava, as nearly as could be ascertained, is 5,175 feet; the thickness of the 

 volcanic capping is, therefore, nearly 600 feet. The lava is an andesite, according to Mr. Wads- 

 worth's determination. It is frequently spoken of as " gray lava," to distinguish it from the black 

 basalt which is found upon some of the neighboring ridges. 



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