40 ANNUAL ADDRESS, MDCCCLXXIII. 



opportunity of comparing them side by side with their 

 congeners of cretaceous times. 



I have now shown the distribution of limestone through- 

 out the successive geological epochs. My brief description 

 would, however, be incomplete if I omitted to mention 

 the views of Mr. Hull, M.A., F.R.S., who in a paper in 

 the Geological Magazine for 1869, p. 364, has shown 

 very strong evidence of a ternary classification of rocks. 

 He points out an unmistakable tendency in each geological 

 formation or group of strata to assume a threefold ar- 

 rangement, having sedimentary matter for the upper and 

 lower members, with a central calcareous member inter- 

 posed, and that this arrangement is repeated in all the 

 series from the Laurentian to the tertiary times. From 

 this Mr. Hull argues that geological history has repeated 

 itself in cycles, every epoch having, as at present, deposi- 

 tions of sedimentary matter over wide littoral areas, with 

 the accretion of limestone and calcareous matter in the 

 deep sea, out of the reach of sediments which represent 

 the more littoral conditions, whilst the calcareous represents 

 the pelagic or deep sea conditions of each cycle of time. 

 He thus tabulates the supposed arrangement : 



A NATURAL GROUP. Lower stage, representing prevalence of land with 

 movement, producing chiefly sedimentary strata. Middle stage, repre- 

 senting prevalence of sea with quiescence, producing chiefly calcareous 

 strata. Upper stage, representing prevalence of land with movement, pro- 

 ducing chiefly sedimentary strata. 



This grouping is well represented by the Triassic series 

 with its new red sandstone and Bunter beds for the 

 lower stage, muschelkalk limestone for the middle stage, 

 and red marls and Keuper sandstones for the upper 

 stage. 



