CLASSIFICATION OF MINERAL LANDS. 57 



valleys or canyons from which it is difficult to sight triangulation 

 signals. 



In the above description no mention has been made of the method 

 of determining elevations, which in much of the work are of great 

 importance. "With a telescopic alidade the angle of elevation or de- 

 pression from one point to another can be accurately determined. 

 This angle being known and the distance between the two points be- 

 ing measured with a scale from the locations of the points on the map 

 or by reading a stadia rod, the problem resolves itself into the solu- 

 tion of a right-angle triangle in which one side and one of the acute 

 angles are known, the side to be determined representing the vertical 

 distance between the two points, or, in other words, the difference of 

 elevation between them. The altitude of a starting point being 

 known or assumed, it is possible to calculate the altitude of any num- 

 ber of points in any direction from it. Elevations determined by thi^ 

 method are not absolutely exact but are of sufficient accuracy for 

 most purposes of geologic mapping. In ordinary practice they arc 

 correct within 5 or 6 feet, but with certain refinements of work the 

 error can be reduced to 1 foot or even less. 



It is advantageous to represent on a map in some manner the form 

 of the rock folds, which may or may not correspond to the surface re- 

 lief. Ordinarily surface relief is represented on a contour map by lines 

 drawn to indicate definite elevations above sea level, one line, for ex- 

 ample, at 4,000 feet, the next at 4,100 feet, the next at 4,200 feet, and 

 so on. These lines not only show the configuration of the surface but 

 also give approximately the elevation of any point within the area 

 thus mapped. It is possible to contour in the same way the surface 

 of an oil-bearing sandstone or a coal or phosphate bed which has 

 been bent into a fold. Such contours are known as structure con- 

 tours. They bear no necessary relation to surface relief. In fact, 

 a valley may occupy the axis of an anticline or a ridge the axis of a 

 syncline. 



Sedimentary beds deposited one above another during a period in 

 which there has been no earth movement are parallel — that is, if 

 one sandstone so deposited is 2,000 feet above another at one place 

 it is probably 2,000 feet above the other 1 mile or 5 miles away. If 

 since their deposition the beds have been tilted from the horizontal, 

 the dip of a bed at the surface in general represents the dip of the 

 beds below it. If a certain oil-bearing sandstone dips 5° E. where 

 it appears at the surface and overlying beds of the same formation 

 outcropping east of the exposure of the first bed also dip 5° in the 

 same direction, it is probable that the first bed and all intermediate 

 beds have the same dip in this area also. A dip of 1° is equivalent 

 to about 92 feet in a mile, and a dip of 5° to about 460 feet in a mile. 



