THE VICINITY OF LA PORTE AND GIBSONVILLE. 447 
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. These 
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 
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. Silicitied 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 
elay-like, showing fora 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 
suft 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 
line of demarcation between the soft and the hard rock is always sharp and distinct, even wnen 
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. This 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 I]linois Hill. 
The bed-rock at this hill is from fifty to one hundred feet higher than that in the adjacent Secret 
Diggings mines. The diagram (Plate 8S, 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 
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 the 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 coated 
with a red ora black scale, and were brittle, as if they had been exposed toa high heat. Ina 
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 
