122 UNIVERSITY OF COLORADO STUDIES 



mined with difficulty, since the microlites are by no means distinct in 

 their boundaries and are more or less altered. Flow-structure may be 

 seen in the freshest specimens, often distinctly showing near the pheno- 

 crysts about which the microlites curve. 



Trachytic Phase. — About a mile northeast of Sugarloaf Mountain, 

 while the dike at the center is only slightly different from that on the 

 peak, it grades toward the walls into a decided trachyte. The dike is 

 exposed in prospect holes and is probably not over 20 feet wide. The 

 latite at the center occupies about one- third of the entire width, and 

 while it is too badly decayed to permit certain identification of all the 

 original constituents, it seems that orthoclase is more important and 

 ferromagnesian minerals slightly less abundant than on the peak. The 

 marginal rock is found in fresh condition in a tunnel driven on the con- 

 tact with the granite. It contains no ferromagnesian minerals, and 

 apparently differs from the trachyte described above only by the presence 

 of flow-structure and in containing a little more quartz and a few plagio- 

 clase phenocrysts and lacking fluorite. This tunnel has also exposed 

 a zone of friction-breccia and friction- conglomerate nearly three feet 

 wide in which the fragments are principally trachyte. 



Nomenclature. — The following is Mr. Hogarty's analysis of the 

 Sugarloaf rock: 1 



Silica 63 .64 



Titanium dioxide 43 



Alumina 18 .05 



Ferric oxide 2 . 14 



Ferrous oxide 1 • 80 



Manganous oxide 46 



Calcium oxide 3 • 36 



Magnesium oxide 1 • 01 



Sodium oxide 3 65 



Potassium oxide 4-73 



Phosphoric anhydride 18 



Water 1 07 



100.52 

 It appears that the total alkalis and the excess of potash over soda 

 are more characteristic of trachyte than of andesite. Yet the amount of 



' Loc. cit., p. 181. 



