FIELD GEOLOGY. 215 



The geologist having, by careful and accurate obser- 

 vation of all available data, attained this knowledge of 

 a district, and, viewing the whole tract at once, as 

 upon a map, can in his mind lift up, as it were, formation 

 after formation, and see those beneath as plainly as 

 though they had never been covered. In imagination 

 removing one series of deposits he sees beneath it the 

 surface of another, which may or may not have been 

 eroded previous to the deposition of the one thus re- 

 moved. If it has not, he views its full extension, marks 

 its gradually thinning boundaries, defined probably by 

 lines of cliffs and of conglomerates, the ancient beaches ; 

 if denuded, he sees the exposed edges of the eroded 

 strata, cut through by ancient valleys, and in places 

 outliers only remaining to mark its former extension. 

 Some parts of the old surface may be covered by sheets 

 of lava, intersected by volcanic dykes, or perhaps dislo- 

 cated by faults in every direction. He notes also the 

 gradual approach of those variations in the conditions 

 of sea, land and climate which ultimately lead to the 

 deposition of the succeeding formation, and which drive 

 before them, slowly but inevitably, the existing forms of 

 life and bring in new races of inhabitants. 



When the mind has grasped such facts as these, facts 

 supported it may be by strongest evidence, the aim and 

 end of our geological research has been mainly achieved. 

 We can not always hope to carry our inductions to the 

 extent here indicated, but it may done with very fair 

 probability of accuracy within reasonable limits. And 

 it is to this kind of knowledge that Geology must look 

 for its practical result, in the assistance that it affords 

 to mining, engineering, quarrying, and well-boring ope- 



