loS The Amenca7i Geologist. Febmary, 1899 
The age of these pegmatyte dikes in different parts of the country 
varies considerably. In crystalline rocks exposed in the lower part 
of the grand canyon of the Colorado in northern New Mexico, the 
dikes- break through the granitic rocks, but come unconformably 
against the base of the Algonkian series there, and are consequentlj' 
pre-AIgonkian in age. All of the larger dikes observed in the Rockj- 
Mountain region have been to a greater or less degree involved in 
the schistosity and other structural modifications of the crystalline 
rocks, and consequently must have been formed either prior to or 
during the earlier stages of the uplift of these mountains. In the Ap- 
palachian region these dikes are not in most cases extensively involved 
in the schistose structures of the rocks, but in some instances they 
have undergone considerable changes in connection with the produc- 
tion of these structures. The condition of the material in the dikes 
seems to indicate that they were formed either prior to, or during, 
the early stages of the uplift of these mountain regions as in some 
cases they are involved in folds quite similar in general character to 
those typical of the Appalachian structure. 
Occasionally the sheets of mica have themselves been folded under 
pressure, but as a rule they show no such disturbance, having been, 
like the coarse feldspar and quartz of the dike, but little modified in 
connection with the mountain uplifting. These blocks of mica, how- 
ever, frequently have their commercial value in large measure de- 
stroyed by the reproduction of what is called "rolled" or "ribbon" 
mica, the sheets of mica being cut into narrow strips with parallel 
edges. These edges of the rolled mica appear in all cases to be paral- 
lel to certain axes of crystallization and the cause of this "ruling" 
and the conditions under which it has been produced are not well 
understood. 
The following papers were read by title: 
Stratigraphy of the Pottsville Series in Kentucky; by Marius R. 
Campbell, Washington, D. C. 
American Homotaxial Equivalents of the Original Permian; by 
C. R. Keyes, Des Moines, Iowa. 
Geology and Physiography of the West Indies; by Robert T. -Hill. 
Washington, D. C. 
Surface Features of Northern Kentucky; by Marins R. Campbell. 
Washington, D. C. 
Geology of the cry.stalline rocks of Manhattan Island and Vi- 
cinity; by F. J. H. Merrill, Albany, N. Y. 
Conditions of Formation of Dykes and Vein Fissures; by N. S. 
Shalcr, Cambridge, Mass. 
Origin of the Highland Gorge of the Hudson River; bv F. J. H. 
Merrill, Albany, N. Y. 
The lowan Drift; by Samuel Calvin, Iowa City, la. 
Loess Deposits of Montana; by N. S. Shaler, Cambridge. IMass. 
Spacing of Rivers with Reference to the Hypothesis of Base- 
leveling; by N. S. Shaler, Cambridge, Mass. 
