THE COMSTOOK MINES. 105 



very well and are maintained in good condition. When movements of the 

 ground force any part of the work out of line, the disturbed sets may be taken 

 out and replaced by new, or re-adjusted without difficulty; and, unless the 

 ground is very bad, with a tendency to move in large masses, the perpen- 

 dicular line of the shaft may be well preserved. 



When it becomes necessary to retimber a shaft or any considerable part 

 of it, the work is usually carried on in one compartment or one-half of the 

 shaft, while the remaining compartments are kept for use in the ordinary 

 operations of the mine. 



Nearly all of the deep shafts of the Comstock mines have required more 

 or less retimbering ; this has been especially necessary in the deeper portions, 

 near the vein, where the ground passed through is sometimes very heavy, 

 consisting of massive clay, which exerts a tremendous pressure upon the 

 timbers. To withstand this latter it is sometimes necessary to resort to 

 methods still more substantial than that just described. A portion of the 

 Yellow Jacket shaft, passing througlj uncommonly heavy ground, has been 

 lately timbered with douBle sets, an outer set inclosing the ordinary single set, 

 giving additional strength to the frame. 



The deep shafts of the Gould and Curry, Savage, and Chollar-Potosi 

 mines have all encountered very bad ground in depth, involving large expense 

 for the proper maintenance of the work, 



Several hundred feet in each of these shafts have been lately retimbered 

 with 14-inch timbers, placed in sets not four feet apart as just described, but 

 close together, making a solid casing 14 inches thick. In the Chollar-Potosi 

 shaft the sill-pieces are made in two parts of unequal length, one long and one 

 short piece, and in each succeeding set these pieces are so placed as to break 

 joints, the joint occurring opposite one or the other of the timber partitions 

 between the conipartments. The corners of the frame are joined together 

 simply with a mitred joint as shown in Fig. 1, Plate II; and the end pieces 

 are placed transversely betv/een them, in such manner that the end of each 

 transverse piece bears with half its thickness against two adjacent sill-pieces. 



Figs. 4 and 5, on Plate II, illustrate this method of construction. Fig. 4 



is a longitudinal section of a portion of the shaft, showing one end and two 



partitions. The sill- timbers are laid close together, one upon another: the end- 

 14 



