52 FIELD GEOLOGY. 



The foregoing table has been prepared for the pur- 

 pose of getting an approximate idea of the rise or fall 

 of the ground in a certain distance, or of the depth to 

 which a bed with ascertained dip may have descended 

 at any particular spot, and the consequent thickness of 

 the beds above it. The centre column gives the angle 

 of inclination of the beds or of the surface, the columns 

 on the left hand the proportion of rise or fall, as the 

 case may be ; those on the right the vertical depth to a 

 bed at 100 units from its outcrop, and the actual thick- 

 ness of the beds above, supposing them to have been in 

 their original plane of stratification. These 100 units 

 are measured along a horizontal surface in the direc- 

 tion of its dip, and allowance has of course to be made 

 for rise or fall of the ground, if uneven between the 

 points. 



Exposed Sections. In the examination of the rocks 

 exposed in quarries, pits, and so on, we make a definite 

 series of observations. On entering we should probably 

 have in our mind a general idea of what we are going to 

 see (except in a district quite new to us), from former 

 experience of the rocks in the neighbourhood, or from 

 what might otherwise be reasonably expected. We 

 should also infer from the kind of excavation in what 

 material it had been made as, for instance, a brick-yard, 

 which would be in clay or loam of some kind ; a lime- 

 kiln in limestone, whether of compact nature as the 

 41 Lias," or earthy as the " Chalk." In a glance round 

 the excavation we get further ideas of the general 

 character and lie of the beds, and we then proceed to 

 make more detailed examination. 



Note. Eailway cuttings often afford excellent 



