CHO1CK IIKTWKKX A TrXXKL AND AN ni'KN (II 3 



grade, and there the tunnel should begin. If, therefore, we 

 draw on the longitudinal profile of the tunnel a line parallel to 

 the plane of the tracks, and 60 ft. above it, this line will cut 

 the surface at the points where the open-cut approaches should 

 cease and the tunnel begin. This is a rule-of-thumh determi- 

 nation at the best, and requires judgment in its use. Should 

 the ground surface, for example, rise only a few feet above the 

 60 ft. line for any distance, it is obviously better to continue 

 the open cut than to tunnel. 



THE METHOD AND PURPOSE OF GEOLOGICAL SURVEYS 



When it has been decided to build a tunnel, the first duty 

 of the engineer is to make an accurate geological survey of 

 the locality. From this survey the material penetrated, the 

 form of section and kind of strutting to be used, the l>est form 

 of lining to be adopted, the cost of excavation, and various 

 other facts, are to l>e deduced. In small tunnels the geological 

 knowledge of the engineer should enable him to construct a 

 geological map of the locality, or this knowledge may IK- had 

 in many cases by consulting the geological maps issued by tin- 

 State or general government surveys. When, however, the 

 tunnel is to be of great length, it may 1x3 necessary to call in 

 the assistance of a professional geologist in order to reconstruct 

 accurately the interior of the mountain and thereby to ascer- 

 tain beforehand the different strata and materials to U 

 excavated, thus obtaining the data for calculating both the. 

 time and cost of excavating the tunnel. 



The geological survey should enable the engineer to dcicr- 

 mine, (1) the character of the material and its force of cohe- 

 sion, (2) the inclination of the different strata, and (8) the 

 presence of water. 



Character of Material The character of the material through 

 which the proposed tunnel will penetrate is liest ascertained 

 by means of diunond rock-drills. These machines bore an 



