GEOLOGY. 



Fig. 65. 



and draw the line A B from southwest to northeast 

 through the spot we select for our shaft, and then, with 

 some leveling instrument, ascertain the undulations of 

 the ground along that line, taking the level of the sea as 

 our c datum line.' Having thus got the true outline of 

 the ground, which we may suppose to be given in Fig. 

 65, we draw lines inclined at an angle of 20 toward the 

 southwest at the several spots where the section line is 

 cut by the outcrops of the beds, or by straight lines join- 

 ing them, and this section will then show us the depth 

 at which the shaft will reach any of the beds. Similar- 

 ly, if we wish to know the thickness of the whole suc- 

 cession of beds, from the highest to the lowest, of those 

 exposed in the district, the length of the line marked 

 thickness will give it to us, when measured on the scale 

 we adopt for our map and section." 



In these simple operations is indicated the mode of 

 ascertaining some great geological facts, for it is by such 

 observations and measurements that we gain a far more 

 extensive knowledge of the crust of the earth than we 

 can by the deepest mining excavations. The great up- 

 heavals have turned up to us the leaves of geological 

 history for our reading, many of which we could not 

 otherwise have read, for they would have been so deep 

 that no exploration of man could have reached them. 



222. Conformable and Unconformable Strata. Strata 

 are said to be conformable when their surfaces or planes 



