VEIN MATERIALS IN ERUPTIVE ROCKS. 



577 



The discrepancies in these were not such as to have afifected the final con- 

 clusion, but it was considered best to publish only those whose positive 

 results were beyond a doubt.' Comparative!)' few sedimentary rocks were 

 tested, for the reason that in all cases where not evidently exposed to the 

 influence of the ore currents they were found to be barren of all the essen- 

 tial vein materials. 



Baryta determinations. — Baryta forms an essential constituent of one vari- 

 ety of feldspar (hj^alophane), which contains from 9 to 20 per cent, of it, 

 and it has also been found in small amounts in andesine, oligoclase, and 

 orthoclase feldspars, where it is generally associated with strontia; probably 

 both might have been found much more frequently if analysts had made 

 a special search for them. As these substances are generall)' recognized as 

 possible constituents of eruptive rocks, it was not considered necessary to 

 make a great many tests in order to establish the possibility that the barite 

 in the Leadville deposits might have been derived from the neighboring 

 rocks. As strontium replaces barium to a certain extent in the Leadville 

 mineral, its occurrence is also of significance. Neither of these substances 

 was found in the sedimentary rocks anah'zed, but from eruptive rocks were 

 obtained as follows: 



Barium and stroiilium in eruptive rocks. 



In one specimen each of Mount Zion Porphyry, porphyrite, and rhy- 

 olite, neither baryta nor strontia was found. Thus out of eleven specimens 

 tested five contained baryta, five strontia, and in three neither was detected. 



Lead determinations. — Lead lias also been detected in feldspars, though less 

 frequently than baryta. 



'For this reason the averages vary slightly from those assumed at the time the abstract of this 

 report was written. 



MON XII 37 



