BLACK HILLS GEOLOGY. 313 



authoritative data as to the grade of these ores, but they are 

 reported to yield high values, frequently over ^loo per ton. In 

 the case cited below the value in gold is unusual. They are 

 to be regarded as brecciated zones silicified by solutions, which 

 owe their activity possibly to the influence of the eruptive mass 

 of Ragged Top Mountain. 



The chemical character of the ore will appear from the ac- 

 companying analysis taken from the paper by Professor Smith : 



Moisture o. 1 1 o 



Volatile matter 0.802 



Silica 90.990 



Alumina 2.970 



Ferric Oxide 3-024 



Calcium Oxide 1-138 



Magnesium Oxide trace 



Tellurium 29.26 oz. per ton. 

 Gold 17.34 " " " 



Silver 1.2 1 " " " 



Total 99.034 



Combining the gold, silver and tellurium in the above an- 

 alysis we find them existing in the following relative proportions, 



Tellurium : 61.20 



Gold Z^-'2-7 



Silver 2.53 



Total 100.00 



The Ulster Mme. — In the Ulster Mine the ore occurs in con- 

 tact zones, between the limestone and a very irregularly intruded 

 mass of porphyry. This is cut by a dike of dense green 

 phonolite, and the ore seems to have resulted from the silicifi- 

 cations of brecciated limestone, which has been fractured by 

 the intrusion of Twin Peaks and other porphyry bodies in the 

 Cambrian below. Brilliant purple fluorite occurs in great 

 quantities. The ore is irregularly distributed. It may thin to 

 a mere streak, and again open out to a very large and thick 

 Annals N. Y. Acad. Sci., XH, December 18, 1899 — 20. 



