STKUCTUKE OF EUBY HILL. 27 



foot-wall side of the fissure and shale the hanging wall above the fifth level. 

 Below that level stratified limestone intervenes between the fissure and the 

 shale. In this mine the fissure is generally filled with a hard black clay, 

 and is often not more than an inch wide. It might be mistaken for an 

 ordinary slip, were it not for the differences in character everywhere exhib- 

 ited by the rocks on each side of it. Splices or small slips are of frequent 

 occurrence in connection with it, as is often the case with such fissures. 

 In this mine it appears at first sight to be of little importance, and has been 

 overlooked almost entirely by the engineers who have examined the under- 

 ground workings. When, however, it is taken in connection with its exten- 

 sion through all the mines to the southeast, and with the fact that it is a 

 fault plane along which the whole southwestern country has been raised 

 from 500 to 2,000 feet, it becomes of great importance as regards the struct- 

 ure of Ruby Hill. 



Detailed description of the main fissure. — It will be seen on examining- the plan of con- 

 tacts (Plate III.) and the various horizontal sections (Plates XIII. and XIV.) 

 of the different mines that there are a number of places on the various levels 

 where the distance between the points at which the fissure has been laid 

 bare is very considerable. The usual method of prospecting in the mines 

 southeast of the Richmond has been to run a main level along the line of 

 contact between quartzite and limestone, sometimes cutting through the 

 quartzite where its projections into the limestone are so great that the length 

 of the drift would be materially increased if this contact were followed; and 

 then if ore was not encountered along this line, to seek for it by driving 

 numerous cross-cuts towards what was supposed to be the shale, but was in 

 reality the Ruby Hill fault. Sometimes it was found to be more convenient 

 to keep the principal drift entirely in limestone and cross-cut in opposite 

 directions from it. Often these cross-drifts did not reach either the quartz- 

 ite or main fissure, though the mine superintendents were usually more par- 

 ticular about a thorough exploration of the quartzite contact than they were 

 about the fissure. Drifts along this fissure were uncommon, except in the 

 lower levels, where the quartzite and fissure came together. Owing to this 

 method of prospecting, the plane of the fissure was not as well explored as 

 was that of the contact of quartzite and limestone. There can be no rea- 



