256 



J. H. COLLEfS ON THE GrEOLOGT 



It is obvious that any movemeat of one side of a fissure which is 

 not a perfect plane, must result in bringing into juxtaposition pro- 

 jecting portions of the opposite walls, in whatever direction the 

 movement may have been ; and consequently in forming irregular 

 cavities of greater or less dimensions*. 



The cavities having been thus formed, it appears to me highly 

 probable that they have been filled with pyrites by infiltration from 

 the country rocks. Both porphyrj- and slate are almost everywhere 

 impregnated, and the joints are often lined with pyrites, which 

 invariably contains a trace of copper. The structure of the pj^ites 

 masses seems to me to bear out this idea. A somewhat banded 

 arrangement or grain of the pyrites is often visible, running in a 

 longitudinal direction ; and frequently portions of the slate, very 

 highly pyritized, have been detached from the walls, and enclosed 

 within the main mass. Owing to the absence of cavities in the 

 pyritous mass, crystals are exceedingly rare, and sometimes it is ab- 

 solutel}' compact and structureless. 



The " lenticular" deposits of pyrites vary from a few yards up to 

 three quarters of a mile or more in length, and in width from a few 

 inches up to 500 feet. Prequently two or more of these are connected 

 together by a thin vein, almost a thread, of pyritous matter ; and 

 they are generally traversed more or less by veins of richer ore, 

 copper-pyrites, copper-glance, galena, &c., and occasionally also of 

 nearly pure iron-pyrites, as well as by veins of barytes and quartz, 

 these latter running in a direction transverse to the general direction 

 of the mass. 



Character of the Ore. — As the average copper contents of the ores 

 sent to England only vary between 3 and 4 per cent., I might almost 

 say 3| and 3|, from year to year, an idea has been generally enter- 

 tained that the ore deposits as a whole are remarkably uniform in 

 composition. jSTothing could be further from the truth, as was shown 

 by the series of specimens sent to the Madrid Exhibition of 1883, 

 which averaged from 20 to 30 lbs. in weight, and contained from 0-5 

 up to 60 per cent, of copper. The following are the chief varieties 

 of the ores which I have observed : — 



a. Poor sulphur-ore ; almost pure iron-pyrites, but containing 

 under | per cent, each of copper and arsenic. Generally minutely 

 crystalline, but sometimes fine-grained and compact, breaking 

 with an almost conchoidal fracture. Sold as poor ore (" mineral 

 pobre "). 



h. An ore like a, but containing variable quantities of earthy 

 matters, silica, and silicate of alumina, up to 25 or 30 per cent. 

 Valueless (" esteril "). 



c. A substance like h, but distinctly banded in structure, the 

 siliceous matters often rising to 50, 70, or 90 per cent. The pyrites 

 in this substance is generally more distinctly crystallized than in a 

 or h, the forms being the cube, the pentagonal dodecahedron, or a 



* This mode of formation, which is well known to all students of geological 

 mechanics, is very simply explained by De la Beche in his ' Report on the 

 Geology of Cornwall,' &c. p. 317. 



