34 STRUCTURE. 



recognized geological horizon is used as a "key" rock, and its 

 elevation above sea level is determined either by observations 

 made directly upon it or by its interval being computed from 

 other known horizons above or below it in the rock column. 

 The Pittsburgh Coal is the most important as well as one of 

 the most easily recognized horizons in the Lewis and Gilmer 

 area, and has therefore been used as the key rock for deter- 

 mining structure. The elevation of the base of this coal above 

 sea level is shown on Map II, which accompanies this Report 

 in a separate atlas, by means of green structure contour lines, 

 each of which is plainly marked with figures, to show the 

 height above sea level, in feet, that it represents. Each con- 

 tour line is a line of strike showing that the Pittsburgh Coal 

 is at the same height above sea level at all points through 

 which the line passes. The contour interval is 25 feet, making 

 it possible to know, with reasonable certainty, not only the 

 elevation of the Pittsburgh Coal, but also that of any other 

 formation desired in the rock column at any given point, by 

 merely adding or substracting its known interval from the 

 Pittsburgh Coal, depending upon whether it is above or below 

 the coal. These elevations were obtained, where possible, by 

 hand level from known points on the topographic map, made 

 directly on the coal. In other cases the aneroid was used and 

 carefully checked at numerous times during the day on spirit 

 level determinations recorded on the map. In western Gilmer, 

 as well as in some other parts of the area, the Pittsburgh Coal 

 was found to be absent, but its horizon makes a broad bench 

 that can usually be followed without much difficulty, thus 

 aiding- greatly in securing observations for the structure map. 

 In regions where this coal was found to be under drainage, its 

 position was often obtained from the records of oil and gas 

 wells. In regions where direct information could not be had 

 on the horizon of the coal itself observations were made on 

 whatever other coals or formations could be found above 

 drainage and recognized, and the position of the Pittsburgh 

 determined from these. There is no large variation in the 

 thickness of the several formations ; viz, Monongahela, Cone- 

 maugh and Allegheny, in this area, but in order to secure the 

 best possible results numerous vertical sections were made at 



