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THE VICINITY OF LA PORTE AND GIBSONVILLE. 



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These 



to have crossed to Secret Diggings, the bed-rock is covered to a great extent by slides of earth, 

 and it is not possible to tell the precise position of the deepest portions. As nearly as I could 

 judge, the deep rock has an altitude of only a little more than 4,900 feet, hardly as high as the 

 rock at the upper end of Secret Diggings, where I made the altitude to be 4,913 feet, 

 measurements were made with the aneroid barometer, but in both cases there was a direct com- 

 parison of the aneroid with the mercurial barometer at the Union Hotel, within less than two 

 hours of the time of observation. There is thus a little uncertainty as to the precise mode of con- 

 nection between La Porte and Secret Diggings. There is apparently a low ridge of bed-rock 

 between the places, which, however, does not rise high enough to shut off all possibility of con- 

 nection, and there can be no doubt that a connection formerly existed. The altitude of the bed- 

 rock at the southwestern end of Secret Diggings I made to be 4,828 feet, which gives a total fall 



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of two hundred and forty -nine feet in a little more than two miles. 



The thickness of the gravel has been on the average between forty and sixty feet. I could not 

 learn of the existence of any animal fossils in the gravel. Siiiciiied and carbonized wood have 

 both been found in great abundance. The latter has been used as fuel in the blacksmith-shops. 



The bed-rock under the gravel is in some places a hard bluish slate, but in others it is soft and 

 clay-like, showing for a variable depth alternations of color and arrangement similar to those which 

 have been referred to already in connection with Plug Ugly Hill, near Dutch Flat. Below the 

 soft stratum the rock is moderately hard and bluish in color, resembling in composition and 

 structure a chloritic slate. The depth to which the alteration extends is from nothing to eight or 

 ten feet. The altered rock is grayish, bluish, reddish, yellowish, or nearly white in color. The 

 lino of demarcation between the soft and the hard rock is always sharp and distinct, even when 

 wavy and uneven, and the stripes of differently colored clays run parallel with this line. The 

 planes of cleavage or lamination are independent of the color, and the joints and fractures pass 

 without interruption indiscriminately through all the varieties. In some places this altered rock 

 shows a fine horizontal lamination ; and, again, when seen in mass, it looks as if it were made up 

 of brightly colored concentric shells or layers, resembling the structure of agate. Tins appearance 

 is probably the result of the gradual decomposition of bed-rock boulders. 



The evidences of disturbance subsequent to the deposition of the gravel are of various kinds. 

 Allusion has already been made to the cutting off of the La Porte gravel under the lava ridge to 

 the north of the town. I will now call attention to one or two other cases. The eastern portion of 

 what is included on the map (Plate R) under the name of Secret Diggings is known as Illinois Hill. 

 The bed-rock at this hill is from fifty to one hundred feet higher than that in the adjacent Secret 



The diagram (Plate S, Fig. 2) shows an east and west section across Illinois Hill 

 and Secret Diggings as sketched on the spot. No accurate measurements were taken either for 

 horizontal distances or for altitudes. The top of the gravel which remains unwashed at the north- 

 ern end of Illinois Hill has an altitude of 4,993 feet, and that of the slate bed-rock at the base of 

 the lava knob is 4,916 feet. The lava knob here referred to covers an area of two or three hundred 

 feet square, and rises to a height of about sixty feet above the adjacent slate. It stands nearly 

 opposite the middle of the Secret Diggings mines. Upon its northern, eastern, and southern sides 

 there used to be a deposit of clean quartz gravel, the greater part of which has been washed away. 

 Its western side forms a part of the steep, precipitous eastern rim of Secret Diggings. The bed- 

 rock in Secret Diggings is said to have been much disturbed and broken up where it abutted 



Diggings mines. 



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against the lava; but I saw nothing unusual either in the position or in the character of the slates 

 where they joined the lava on Illinois Hill. Enclosed within bhe lava at intervals, even to its top, 

 there were small streaks and bunches of quartz gravel, ranging from one or two up to eight or ten 

 feet in the longest dimension. The quartz pebbles were, many of them, partially or entirely coaled 

 with a red or a black scale, and were brittle, as if they had been exposed to a high heat. In a 

 conversation about this (or some other) body of intrusive lava at Secret Diggings, Colonel B. F. 

 Baker of Gardner's Point told me that in early days, when the lava was first struck, the miners ran 

 a tunnel into it at bed-rock level, and then sank a shaft sixty feet in depth without again reaching 



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