Quarrying Shale by the Tunnel System (0fe1a3d8-078e-4519-8659-961800faf264)

Discussion of the paper of DWIGHT T. FARNHAM, presented at the Pittsburgh meeting, October, 1914, and printed in Bulletin No. 93, September, 1914, pp. 2357 to 2364. DAVID T. DAY, Washington, D. C.-Do you know anything about the character of that clay as to organic matter? H. RIES, Ithaca, N. Y.-No; it is a dark brownish clay, sandy in its nature. The brownish coloring is probably due to organic matter. Some lignite is also present. DONALD W. Ross.*-I have worked at the plant under consideration. I have not seen Mr. Farnham's paper; but in answering the question as to the organic matter, I might say there is practically no organic material scattered through the shale. There are, however, several layers of carbonaceous shale and rather low-grade coal interbedded with the formation. There are two very serious drawbacks-one which threatens to shut down the plant and another which merely means the expenditure of money. The first is taking care of the sediment from the washing away of the overburden. The government is doing extensive work at different points about Lake Washington with a view to creating a fresh-water harbor for Seattle. I understand that they keep at the Denny-Renton people to keep the channel of Cedar River clear. The Denny-Renton Co. can hardly help filling up the channel so long as they continue their hydraulic work, so this is probably one of their most serious problems. The other is this: There is a large percentage of what I should call micaceous shale or sandstone-a sandstone which contains a large amount of light-colored mica. Of course only a limited amount of this can be used in the manufacture of paving brick. As they shoot down the bank after the overburden of gravel and loam is washed away, these large boulders of micaceous sandstone come down. The strata have apparently been crushed where the clay pit is located so that the exact formation cannot be readily determined. These pieces of sandstone, 5 or 6 ft. in diameter, have to be blasted and removed from the shale. They bother both in the open pit and in the tunnel mining. In the tunnel mining of the shale blocks frequently cover the holes in the roof of the tunnel through which the shale is loaded into cars.