150 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 23, 



at a certain depth. Everything seems to show that the various 

 deposits occupy the gaping mouths of a fissure which is of moderate 

 dimensions helow, although expanding, and containing much rich ore 

 near the surface. 



Breadth of or ey ground. — The magnitude 'of the deposits of ore 

 is often so large, that a breadth of ground amounting to from twenty 

 to forty feet required to be removed, leaving only very small and 

 insufficient arches. 



Timbering on the grandest scale has therefore been resorted to, to 

 prevent accidents ; and it is not unusual to see pairs of spars, con- 

 sisting of trees hewn square and having a section eighteen inches 

 square lashed together, placed to meet at an obtuse angle midway, 

 and strengthened with diagonal bracings, forming a solid construc- 

 tion, like the vaulted roof of a cathedral. Those only who are 

 conversant with the difficulty of introducing such spars into the 

 small shafts of a mine, and handling them underground so as effec- 

 tually to serve for the purpose intended, and preserve the mine for 

 years vdthout danger or accident, can do justice to the science and 

 engineering that have been brought to bear on this part of the 

 mining operations of the Cobre Company, and it is only a just tri- 

 bute to the skill and activity of the successive managers (all, I 

 believe, of Cornish experience) to allude to so important a part of 

 the oeconomy of these remarkable mines. 



Mundic. — Within the various parts of the lode are found at inter- 

 vals, and at all depths, large quantities of mundic, or iron pyrites, 

 often highly crystalline. Much of this occurs between the north 

 and south deposits of ore ; and, in addition to the masses, and de- 

 tached crystals, the copper pyrites are not unfrequently so completely 

 coated with this worthless mineral as to render it impossible to esti- 

 mate their value by the eye. This is the case more in the upper 

 than in the lower levels. 



Veinstone. — The Cobre mine is open at present to the 160-fathom 

 level below adit ; and, although a fair proportion of reserves exists in 

 the lower levels, it will be readily understood by those accustomed 

 to mining in rich veins, that only a few arches of ground have been 

 allowed to remain in the upper part of the mine. The communica- 

 tions between the different courses of ore at the different depths, and 

 the cross cuts out of the lode to north and south, at various points, 

 as well as the appearance of the lode, where now being removed, are 

 all extremely interesting and instructive. The enclosing country, and 

 the ground between the courses of ore, appear to become more com- 

 pact and regular in descending, and are occasionally very hard. At 

 the 140-fathom level the lode becomes gypseous, considerable quan- 

 tities of white alabaster appearing without sensibly affecting the value 

 of the ore ; but the veinstone a little below becomes very hard, and 

 in some parts of the lode the ore ceases altogether in the vicinity of 

 the gypsum. 



At the lower levels, the heaves and cross courses appear to have 

 rather more influence than near the surface, at least with regard to 

 the quantity of ore. 



