MICA DEPOSITS OP THE UNITED STATES 503 



less than 2 per cent, has a commercial value as sheet mica, the remainder being 

 either thrown away as waste material or pulverized for commercial uses. 



The age of these dikes in different parts of the country may be said to vary con- 

 siderably. In crystalHne rocks exiDOsed in the lower part of the Grand canyon of 

 the Colorado in northei'n Arizona, the dikes break up through the granitic rocks, but 

 come unconformable against the base of the Algonkin series there, and are conse- 

 quently pre-Algonkin in age. All of the dikes observed in the Rocky Mountain 

 region have been to a greater or less degree involved in the schistosity and other 

 structural modifications of the crystalline rocks, and consequently were probably 

 formed during tlie earlier stages of the uplift of these mountains. In the Appala- 

 chian region these dikes are not in most cases extensively involved in the schistose 

 structures of the rocks, but in some cases they have undergone considerable 

 changes in connection with the production of these structures, and the conditions 

 surrounding these dikes seem to indicate that they were formed during the later 

 stages of the uplift of this mountain region, though in some cases the smaller 

 dikes are involved in folds quite similar in general character to those typical of the 

 Appalachian structure. In cases of these larger Appalachian dikes, however, the 

 crystalline character is such as to indicate few or no great pressure changes, and to 

 suggest the association of such dikes with the later development of this mountain 

 region. 



Occasionally in the Black hills and in New Mexico, and less frequently in the 

 Appalachian region, the sheets of mica have themselves been folded under press- 

 ui'e, but as a rule they show little such distortion, having been, like the coarse 

 feldspar and quartz of the dike, but little modified in connection with the moun- 

 tain uplifting. These blocks of mica, however, frequently have their commercial 

 value in large measure destroyed by the reproduction of what is called " ruled" 

 or "ribbon" mica, the sheets of mica being cut into narrow strips with parallel 

 edges, the line of ruling or cutting appearing in all cases to be parallel to certain 

 axes of crystallization; but the cause of this "ruling" and the conditions under 

 which it has been produced are not yet well understood. 



During the reading of Professor Holmes' paper the Fellows in attend- 

 ance at the meeting gathered in the Petrographic Section, which was 

 then regarded as the general Society meeting, with Vice-President Emer- 

 son in the chair. 



The scientific work was declared completed. 



Mr J. S. Diller presented a resolution of thanks to the President and 

 Trustees of Columbia University for the welcome to the University and 

 the use of the rooms and facilities, to the resident Fellows who had 

 served as a local Committee of Arrangements, and to Mr George F. Kunz 

 for his labors in connection with the annual dinner. 



The Society then adjourned. 



