USEFUL METALS 



119 



The ore therefore seems to have IHTM derived from the porphy- 

 ries by leaching and deposited in the limestone by metasoni.it i<- 

 replacement. The aqueous solutions traversing the joint planes 

 and bedding planes of the limestone deposited the ore in later 

 Cretaceous times. 



Newbery and LeConte have suggested that the ores were derived 

 from ascending solutions bearing lead salts or ores consisting of 

 < t russite (the carbonate) , galenite (the sulphide), and anglesite 

 (the sulphate) of lead (Fig. 77). 



Aspen is another important locality in Colorado in which is 



FIG. 77. General view of Rico, Colorado, and Enterprise group of 

 mines. (By permission of the Macmillan Company, from Ries' Economic 

 Geology.) 



found argentiferous lead ores. According to H. Ries, the ores are 

 oxidized and occur in folded and faulted Carboniferous limestone, 

 although the section involves rocks ranging from the Archean to 

 the Mesozoic in age. Two quartz porphyries, one at the base of 

 the Devonian and the other in the Carboniferous at present ap- 

 pear to bear no special relation to the ore bodies. 



At the close of the Cretaceous the rocks were folded into a great 

 anticline, with a syncline on its eastern limit. Contemporaneous 

 with the folding there were produced two faults parallel with the 

 bedding of the Carboniferous dolomite. At the same time much 



