UNCONFORMITY AND CHANGE IN LIFE. 343 



find the Greensand of Dorset and Devonshire resting unconformably 

 on the oolites and older rocks. 



About 1 8 per cent, of the Lower Greensand species pass up into 

 the Gault, but 40 per cent, of the Gault species pass into the Upper 

 Greensand. The Upper Greensand has 20 per cent, passing into the 

 Chalk Marl. The Chalk Marl has 59 per cent, passing into the 

 Lower Chalk, and the Lower Chalk has 3 1 per cent, passing into the 

 Upper Chalk. It is doubtful if any species really survive from the 

 Chalk into the Tertiaries. 



Breaks in the Tertiary Strata. In this country the physical 

 break between the Secondary and Tertiary strata is of the largest 

 kind, and presumably consisted in an upheaval of the sea-bed, so that 

 what had previously been an area covered with organic deposits 

 free from sediments was succeeded, after a certain amount of denuda- 

 tion, by sediments derived from crystalline rocks. But though the 

 Tertiary rocks repeat in their subdivisions differences in life as re- 

 markable as those which have been mentioned already in the succes- 

 sive beds of the Primary and Secondary series, and further contain 

 conglomerates, fresh-water deposits, and several zones rich in land 

 vegetation, indicating oscillations of level, there are no visible physical 

 breaks till after the close of the Hempstead beds, with which the 

 Lower Tertiaries terminate. Whether any newer deposits were 

 ever accumulated in the British area it is now impossible to determine, 

 but the land may well have been upheaved and dry during the 

 Middle Tertiary or Miocene times. It is certain that an immense 

 physical break is indicated by the interval between the Lower Ter- 

 tiaries and the Upper Tertiary Crags, which exhibit an absolute un- 

 conformity with the strata on which they rest. The Bed Crag rests 

 unconformably on the Coralline Crag, and the Boulder Clay rests un- 

 conformably on the Red Crag. 



Thus the number of physical breaks in the British area is inade- 

 quate to account for the breaks in the succession of life. There is no 

 evidence of denudation which would warrant an explanation of the 

 breaks in life by assuming missing deposits ; and we are hence com- 

 pelled to attribute the seeming breaks, in the main, to disturbances 

 in the relations of land and water in adjacent seas which affected 

 the distribution of life in the region which is now Britain, and at the 

 same time varied the mineral character of the sediments in which the 

 fossils became preserved. 



Table of Disturbance in the British Area. 



The following table prepared by Prof. John Phillips shows the 

 geological periods of many remarkable convulsions in Great Britain, 

 and the places where some of the most considerable effects are 

 manifested : 



