214 FIELD GEOLOGY. 



to be not interbedded with the coal and shales, but an 

 eruptive dyke breaking through the strata without the 

 slightest conformity to their lines of stratification. In 

 mapping the eruptive rocks (and intrusive also until 

 proved to be interbedded) it should be borne in mind 

 that they behave at the surface in an irregular manner, 

 very similar to that of the glacial drifts ; their boun- 

 dary must be closely followed, while the remarks upon 

 the drifts (at pp. 34 et seq.) may in these cases also be 

 found of some advantage. 



Veins. The mapping of lodes or metallic veins is 

 very similar, as these follow an irregular course ; but it 

 will usually be found that they make a surface feature, 

 as an elevation or depression according to the varying 

 hardness and dip of the beds or masses in which they 

 occur. 



CHAPTER II. 



Geological Generalisation and Practical Results Water-supply 



Soils. 



Generalisation. A geological examination of a quarry 

 may seem a trifling matter, and one from which no very 

 grand results can be expected. But the knowledge thus 

 obtained, as pointed out in p. 2, is not simply and 

 strictly a local knowledge, for it extends a long way into 

 and beneath the surface of the earth. We see, there- 

 fore, that examination of several such quarries must 

 make us acquainted with the nature and position of 

 the rocks a long way down over all the intervening 

 area. 



