84 PROCEEDINGS OF THE PHILADELPHIA MEETING 



Discussion 



Dr. C. N. Fenner pointed out that as regards the question of a eutectic com- 

 position of pegmatites several points must be held in mind. In the first place, 

 the solutions from which pegmatites have been deposited undoubtedly con- 

 tained not only water, but other volatile ingredients, each of which would by 

 its presence affect the temperatures at wliich tlie nuartz and feldspar would 

 crystallize out, and also the relative inopoitions in which the two would ap- 

 pear. Moreover, whei-e more than one land of feldspar molecule is present the 

 relations become much more complicated because of the formation of solid 

 solutions, and the .simple eutectic relations no longer hold. 



Mr. J. P. WiNTBiNGHAM asked whether a graphic intergrowth was not to be 

 talien as indicating a eutectic. 



Mr. Johnston in reply pointed out that tliere appears to be no character- 

 istic eutectic structure in silicate systems similar to that well known in metal 

 systems, though such eutectic structure may appeal' in silicate systems con- 

 taining \olatile components, this being a point on which there is not laboratory 

 evidence at present. Again, that though the eutectic is the temperature at 

 which the last of the mixture solidifies, much of the material will in general 

 have crystallized out before ; that the eutectic is merely a special point on the 

 curve, and that in all probability its importance is not so great as has been 

 supposed, especially in view of the frequent occurrence of solid solutions. 



Further remarks were made by W. Cross, W. H. Emmons, and J. E. 

 Wolff. 



PRIMARY CHALCOCJTE IN THE FLUORSPAR VEINS OP JEFFERSON COUNTY, 



COLORADO 



BY HORACE B. PATTON 



(Abstract) 



Several sharply defined veins, mainly of fluorspar, but also carrying consid- 

 erable amounts of chalcocite and of other sulphides, have been opened in Jef- 

 ferson County, Colorado. The veins have been worked for fluorspar and, to a 

 lesser extent, for chalcocite. These are clean-cut fissure veins in Archean 

 schist and gneiss, occurring in close proximity to granite intrusions. The 

 association of fluorspar and chalcocite is unusual, and in this case the condi- 

 tions .seem to indicate that the chalcocite is of primary origin. 



Presented in abstract extemporaneously. 



RECENT REMARKABLE GOLD "STRIKE" AT THE CRESSON MINE, CRIPPLE 



CREEK, COLORADO 



BY HORACE B. PATTON 



(Abstract) 



A recent very remarkable "strike" of gold telluride ore has been made in 

 the Cresson mine at Cripple Creek. The conditions are very unusual for this 



