Ill] TABLE OF STRATA. 31 



from some cause or other does not form part of our present 

 geological record. 



As a general introduction to geological chronology, a short 

 summary may be given of the different formations or groups of 

 strata, to which certain names have been assigned to serve as 

 convenient designations for succeeding epochs in the world's 

 evolution. The following table (Fig. 3, pp. 32, 33) represents 

 the geological series in a convenient form ; the most character- 

 istic rocks of each period are indicated by the usual conventional 

 shading, and the most important breaks or lacunae in the records 

 are shown by gaps and uneven lines. The relative thickness 

 of the rocks of each period is approximately shown; but the 

 vertical extent of the oldest or Archaean rocks as shown in 

 Fig. 3 represents what is without doubt but a fraction of 

 their proportional thickness. This table is taken, with certain 

 alterations, from a paper by Prof T. McKenny Hughes in the 

 Cambridge Philosophical Proceedings for 1879. Speaking 

 of the graphic method of showing the geological series, 

 the author of the paper says, "It is convenient to have a 

 table of the known strata, and although we cannot arrange all 

 the rocks of the world in parallel columns, and say that ABC 

 of one area are exactly synchronous with A'B'C of another, 

 still if we take any one country and establish a grouping for it, 

 we find so many horizons at which equivalent formations can 

 be identified in distant places, that we generally make an 

 approximation to homotaxis as Huxley called it. The most 

 convenient grouping is obviously to bracket together locally 

 continuous deposits, i.e. all the sediment which was formed 

 from the time when the land went down and accumulation 

 becjan, to the time when the sea bottom was raised and the 

 w^ork of destruction began. In the accompanying table I have 

 given the rocks of Great Britain classified on this system, and 

 bearing in mind that waste in one place must be represented 

 by deposit elsewhere, I have represented the periods of degra- 

 dation by intervals estimated where possible by the amount of 

 denudation known to have taken place between the periods of 

 deposition in the same di8trict^" 



1 Hughes (79), p. 248. 



