552 Notices of Memoirs — H. P. Woodward — Telhiride Deposits. 



to 300 feet from the surface ; the greatest depths to which it has 

 extended in man}'- cases correspond with the richest portions of 

 the lodes. 



In the mineralized portions of the lode, a gi'eat change in its 

 character is to be noticed, and it has, if anything, still less the 

 appearance of a true lode than that nearer the surface ; however, 

 it continues to be equally as rich, but the gold, instead of being 

 free, occurs in the form of a telluride. The ore is a bluish-green 

 siliceous rock, often thickly studded with minute crystals of pyrites, 

 and intersected b}' quartz veins, which are the richest portions of the 

 lode ; but here, as in the decomposed zone, the gold is found to 

 extend often for a considerable distance into the miuei-alized rock 

 upon either side of these veins. 



Portions of this rock, taken from near the quartz veins, often 

 contain large quantities of small crystals of telluride of gold, 

 pyrites, calcite (which also occurs in veins and vughs), and probably 

 serpentine, thus presenting an entirely different composition from 

 the hornblende and felspar rocks which constitute the country rock, 

 but into which it merges so gradually that it is impossible to say 

 where one ends and the other begins. 



The tellurides are met with as veins, splashes, and minute 

 crystals disseminated throughout the entire mass. In the first of 

 these forms, they appear to have been deposited subsequently to the 

 quartz, for the veins intersect the latter, often filling right-angle 

 cracks in them, which, when encountered, make a great show, since 

 they may be seen in places all the way down the side of a shaft for 

 40 or 50 feet as veins several inches in width, but which upon 

 being broken into only prove to be of slight thickness, with quartz 

 behind them. Fortunately this is not invariably the case, for in many 

 instances small solid veins of the tellurides, quite independent of the 

 quartz, have been traced for a distance of 60 or 70 feet. 



Origin of Lodes. — With regai'd to the origin of the lodes, the only 

 explanation that is tenable is that advanced hj Dr. Penrose in his 

 Pieport upon the origin of the Cripple Creek lodes. This is, that 

 a series of fissures has been formed without any yawning, gaping, or 

 faulting, up which highly heated mineral solutions were forced 

 which permeated the country rock on either side of these cracks, 

 dissolving out certain of its constituents, and replacing them by 

 others, thus altering the nature of the rock to a large extent near the 

 fissure, and gradually less further and further from it until no 

 alteration at all has taken place, and the country rock has remained 

 in its original form. 



When a number of these veins are met with, following the same 

 genei-al course, and at no great distance from each other, minerali- 

 zation has taken place over a greater extent, thus presenting the 

 appearance of a large lode, whilst when only one occurs, the zone 

 of alteration is of course limited, and therefore the lode is said to be 

 small or to pinch. It is almost needless to mention that the 

 richest portions are almost invariably met with at the most highly 

 mineralized points, and hence where the lodes are largest. 



