THE GEOLOGY OF BRIDPORT. Ill 



of Germany, which is found in the Alps and the Apennines, as 

 well as in the Jura Mountains, from whence it takes its name, and 

 is also well represented in India and North America, exhibits these 

 differences in every possible variety. It must be clearly understood 

 that it is the study of fossil forms of life which determines the 

 divisions and groupings of the strata, and the various species of 

 ammonites are useful for determining and naming the Palseonto- 

 logical zones. Again, the same place will exhibit the successive 

 deposition of beds identical in appearance, but containing a 

 succession of varying fossil forms of life, or, again, with compara- 

 tively little change in the fossil life we may have a succession of 

 beds greatly differing in the nature of their sedimentary materials. 

 Between the top of the Lias and the top of the Portlandian stage 

 there is in England a five-fold repetition of a three-fold arrangement 

 of Argillaceous, Arenaceous, and Calcareous beds. " Such an alterna- 

 tion of sediments," says Professor Geikie,* "points to interrupted de- 

 pression of the sea bottom. It permitted the growth and preservation 

 of different kinds of marine organisms in succession over the same 

 areas at one time sandbanks, followed by a growth of coral reefs, 

 with abundant sea-urchins and shells, and then by an inroad of fine 

 mud, which destroyed the coral reefs, but in which, as it sank to 

 the bottom, the abundant cephalopods and other mollusks of the 

 series were admirably preserved." Some of these inroads of mud 

 doubtless occurred at the time of spring freshets when quantities 

 of Ammonites and Belemnites and other mollusca would be 

 overwhelmed, buried, and preserved, in the torpid condition in 

 which they had passed the winter. But beyond the effect of such 

 floods as still occur in many parts of the world we have no reason 

 to imagine the existence of catastrophic changes. The funda- 

 mental lesson Geology has to teach us is the vastness of the work 

 that may be accomplished by the steady action of natural causes 

 acting through enormous periods of time ; and the cliffs of Dorset- 

 shire bear impressive witness to the stupendous patience of Creative 

 Energy. 



* Geikie Text Book, p. 774. 



