Ill] GEOLOGICAL SECTIONS. 27 



the deposits of an earlier age. If we could remove all these 

 surface accumulations of sand, gravel, peat and surface soil, 

 and take a bird's eye view of the bare surface of the rocky 

 skeleton of the earth's crust, we should have spread before 

 us the outlines of a geological map. In some places fairly 

 horizontal beds of rock stretching over a wide extent of 

 country, in another the upturned edges of almost vertical 

 strata form the surface features ; or, again, irregular bosses of 

 crystalline igneous rock occur here and there as patches in 

 the midst of bedded sedimentary or volcanic strata. A map 

 showing the boundaries and distribution of the rocks as seen 

 at the surface, tells us comparatively little as to the relative 

 positions of the different rocks below ground, or of the relative 

 ages of the several strata. If we supplement this superficial 

 view by an inspection of the position of the strata as shown on 

 the walls of a deep trench cut across the country, we at once 

 gain very important information as to the relative position of the 

 beds below the earth's surface. The face of a quarry, the side 

 of a river bed or a railway cutting, afford HOKIZONTAL SECTIONS 

 or PROFILES which show whether certain strata lie above or 

 below others, whether a series of rocks consists of parallel and 

 regularly stratified beds, or whether the succession of the strata 

 is interfered with by a greater or less di vergence from a parallel 

 arrangement. If, for example, a section shows comparatively 

 horizontal strata lying across the worn down edges of a series 

 of vertical sedimentary rocks, we may fairly assume that 

 some such changes as the following have taken place in that 

 particular area. 



The underlying beds were originally laid down as more or 

 less horizontal deposits ; these were gradually hardened and 

 compacted, then elevated above sea-level by a folding of the 

 earth's crust ; the crests of the folds were afterwards worn down 

 by denudation, and the eroded surface finally subsided below 

 sea-level and formed the floor on which newer deposits were 

 built up. Such breaks in the continuity of stratified deposits 

 are known as unconformitiks ; in the interval of time which 

 they represent great changes took place of which the records 

 are either entirely lost, or have to be sought elsewhere. 



