2 STRATIGRAPHICAL GEOLOGY 



one thing, and the grouping of such strata into larger systems, 

 representing definite geological periods, is another thing, and one 

 which requires consideration from several points of view. A 

 subsequent chapter, therefore, will be devoted to a discussion of 

 the principles upon which divisional lines may be drawn in 

 any established succession of rock-groups. 



The two principal tests by which the relative age of different 

 strata is determined are (1), Superposition ; (2), Fossil contents. 



The conclusions to be drawn from the superposition of strata are 

 explained in most text-books of physical geology, and the law of 

 vertical succession which forms the basis of the whole fabric of 

 historical geology may be stated as follows : " In any succession 

 of beds each one represents the conditions which prevailed over a 

 certain area for a certain length of time, the lowest is the oldest, 

 the uppermost is the newest, and the relative age of the others is 

 indicated by their relative position." If the strata are inclined, the 

 right order is ascertained by making a geological survey of the 

 district, and constructing a section at right angles to the general 

 strike of the beds. 



Again, by tracing any one set of strata horizontally along their 

 strike from one part of a district to another, where perhaps there 

 are other rocks of a different kind, we obtain a datum of reference 

 by which to determine whether the rocks in the second area are 

 newer or older than those in the first. 



So long as there is physical and geological continuity between 

 the different portions of a district, i.e. so long as some one member 

 of a conformable series of strata can be followed continuously, such 

 a survey generally affords sufficient data for ascertaining the relative 

 position of the rocks which occur in the district, and for constructing 

 a table of their vertical succession. But when either geographical 

 or geological continuity is interrupted that is, when a district or 

 country is separated from others of like structure either by the sea 

 or by tracts of totally different rocks then we must begin all over 

 again, and construct an independent table of strata for the new 

 district. For example, Wales is a district to all parts of which one 

 system of classification and nomenclature can be applied. Similar 

 groups of rocks occur in Ireland on the one hand, and in Cumberland 

 on the other ; but Wales is physically separated from Ireland by 

 the Irish Sea, and geologically separated from Cumberland by the 

 Triassic plains of Cheshire and Lancashire ; hence the same system 

 of nomenclature could not be used in the other districts, the vertical 

 succession had to be determined independently, and local names 

 given to the different subdivisions in each district before they 

 could be in any way compared with one another. 



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