SUMMER MEETING. 69 



la making a geological survey, it is impracticable to make sound- 

 ings for bed rock over au entire proposed site. This determination can 

 be made by a series of levelings. Oiioose some point on the outcrop 

 of the bed-rock. Sometimes bed-rock is obscured by surface debris. 

 If so, it can be located by shallow trenches cut along the slopes at 

 right-angles to the trend of the hills. By bed-rock is here meant any 

 material that is wholly or partly impervious to water. Let us suppose 

 the place of beginning is outside of or near the southwest corner of 

 a proposed site. We assume an elevation of, say, 1000 feet above sea- 

 level. We level in a northerly direction, say, 1200 feet. Here we find 

 the bed-rock to be 15 feet lower than at the place of beginning. The 

 elevation of bed-rock is here, then, 985 feet. We turn east and cross 

 a low hill, or swell. Just over the brow we find bed-rock to be 10 feet 

 higher than at the last point of its cropping. Here its elevation is 

 995 feet. We pass along the brow of the hill, or swell, and tiind the 

 bedrock near the southeast corner of the site to be 25 feet higher 

 than at the last mentioned point — the southeast corner. The eleva- 

 tion of bed-rock at the northeast corner is, then, 1020 feet. Now, it 

 will be seen from this that the bed-rock dips most rapidly to the north- 

 west. At the southeast corner it is 35 feet higher than at the north- 

 west, 20 feet higher than the southwest, and 25 feet higher than the 

 northeast corner. Assuming the parallelogram described by the sur- 

 vey to be about 1000 feet from north to south and 1500 feet from east 

 to west, or to contain about 35 acres, the diagonal or hypothenuse 

 would be about 1800 feet. The dip, then, in going 1800 feet from the 

 southeast corner toward the northwest, is 35 feet, or about 2 per 

 ■centum. In going toward the west, the dip is 20 feet in 1000, or 2 

 per centum. In going toward the north, it is 25 feet in 1000, 

 or 22 per centum. In going from the place of beginning, southwest 

 corner, toward the north, the dip is 15 feet in 1000, or 1^ per centum. 



Now let U3 make a leveling from the southeast toward the north- 

 west. At 500 feet, the surface has an elevation of lOJO. Now going in 

 this direction, the dip is 2 per centum ; 2 per cent of 500 is 10. Ten 

 feet must be subtracted from 1020 feet for the elevation of bed-rock at 

 this point. It is, then, 10 10 feet. If the surface, as calculated above, 

 has an elevation of 1030 feet and the bed-rock at the same point has an 

 elevation of 1010 feet, the bed-rock lies 20 feet beneath the surface, or 

 the difference of 1030 and 1010 feet. 



It would be entirely too tedious to gi<*e more extended calcula- 

 tions. But remember to subtract the per centage of dip if going in 

 the direction of dip, and to add the per centage if going toward a 

 point at which the bed-rock is higher. This subtraction or addition 



