loS Ol'TllMs,,| KIELD-GEOLOG1 



say on tins part <>! inv Mibjrc t, that \\ - |iiirod not 



only to make observation on geological structure, but to 

 formulate them on paper, and to const nut the geological 

 map of a region. 



The usual sign used on geological maps to express the 

 dip of strata is an arrow pointing in the direction of in- 

 clination (the direction being found on paper by help of 

 an ordinary protractor), with the number of degrees of 

 angle shown in figures at the side of it. We. place, 

 therefore, an arrow at each point on the map when- NSC 

 ascertain the dip of strata. A glance at the map (Figs. 

 27 and 28) will show how this is done. Each arrow 

 marks the site of the observation, and with its accom- 

 panying figures records the result. Where possible we 

 enter beside the arrow some symbols, or contracted 

 writing, to describe the nature of the rock, or any other 

 particulars which it seems desirable to record. Further 

 detail, where required, finds its place in the note-book. 



Seltction of Horizons Mapping of Outcrop. As it is 

 impossible on any ordinary map to represent every bed 

 of rock, the geologist must decide what beds should be 

 selected to be traced out. This cannot always be done 

 until considerable progress has been made with the work. 

 The selection must depend not merely upon the geolo- 

 gical or industrial importance of the strata, but also, and 

 not less frequently, upon the extent to which they are 

 exposed and capable of being followed across the district. 

 A particular stratum of no special interest in itself may 

 come to have a high importance as a geological horizon 

 or platform if it is easily recognisable, and from its thick- 

 ness, hardness, or other peculiarity, stands out so promin- 



