THE OEE DEPOSITS. 67 



cal interchange or an actual replacement of the rock-mass in which they 

 were deposited. 



"III. That the mineral solutions or ore-currents concentrated along 

 natural water channels and followed by preference the bedding planes at a 

 certain geological horizon; but that they also penetrated the mass of the 

 adjoining rocks through cross-joints and cleavage planes. 



"And with regard to distribution — 



"I. That the main mass of argentiferous lead ores is found in calcareo- 

 magnesian rocks. 



"II. That the silicious rocks, porphyries, and crystalline rocks contain 

 proportionately more gold and copper." 



As regards origin, the Eureka and Leadville deposits do not differ, ex- 

 cept that in Eureka District the metal-bearing solutions came from below, 

 and their connection with eruptive rocks is not as plain in Eureka as in 

 Leadville. 



As regards their mode of formation, the deposits of the two regions 

 differ onlv in respect to the manner in which the solutions of minerals were 

 distributed. In Eureka, also, the lead is found only in the limestone, and 

 the most silicious rocks cany the most gold. 



The varieties of minerals found in the two districts are similar, but the 

 galena in Eureka seems to have been more completely oxidized than that 

 in Leadville. 



Deposits of Cumberland and Derbyshire. — The lead deposits of Cumberland and 

 Derbyshire in England are found in the Carboniferous limestone, between 

 the strata of which there are masses of porphyry," which in that country 

 are called "toadstone." The ore is found in fissures which cut the strata. 

 With these fissures pipes, caves, and other irregular openings containing 

 ore are more or less closely connected. Flat bodies or beds are also found 

 between the strata. The fissures are mostly occupied by true lodes, which, 

 however, do not contain ore where they traverse the porphyry, and are not 



According to v. Groddeck (Lagerstiitten der Erze, p. 245; it is doubtful whether this porphyry is 

 intrusive or whether the limestone overlying it was deposited after its eruption. Investigation made 

 by Mr. Emmons of the deposits in Leadville, which occur in limestones of the Carboniferous, seem to 

 prove that the porphyry, which in that region is also found between the strata, is intrusive. And 

 although this fact does not absolutely warrant the belief that such is the case with the deposit? in 

 England, it heightens the probability of such a manner of occurrence. 



