518 THE JOURNAL OF GEOLOGY. 
considerations obtain in the selection of photo-topographic sta- 
tions as for trigonometric stations, z. ¢., that the intersections 
for the location of points may be of fairly well conditioned tri- 
angles. Every photograph gives—knowing the focal length of 
the lens—the angular measurement of every point on the photo- 
graph, 7. ¢., the azimuth of any point from any other point, and 
also the angular elevation or depression above or below the hori- 
zon of the camera. Orroy |p Kvorz 
DEPARTMENT OF THE INTERIOR. 
Ottawa, Canada. 
