VI . 



Desert or Steppe Conditions in Britain : a Study 

 in Newer Tertiary Geology. 



THOSE who, without taking an active part in geological research 

 or controversy, attempt to follow the latest results of the science, 

 must often be struck by the curious way in v/hich old theories fade 

 and disappear, without being directly attacked. Such a fate seems to 

 have overtaken the Diluvial Theory, notwithstanding certain recent 

 attempts to revive it ; we would now draw attention to the crippled 

 state of the allied theory of the former existence of a " Pluvial Period." 

 That a period of somewhat heavier rainfall may have existed during 

 some part of newer Tertiary times, we are in no way concerned to 

 deny ; but as a question of evidence it is noteworthy that all the facts 

 formerly relied on can now be shown to bear a quite different inter- 

 pretation, and that the new facts accumulated during recent years 

 tend to demonstrate the former occurrence, in place of the supposed 

 Pluvial Period, of one or more periods of excessive drought. 



Geological text-books still teach that the wide sheets of gravel 

 found in the river valleys of the southern half of England, where only 

 sands and clays are now deposited, point to a former excessive rain- 

 fall. Some of them even still suggest that the rivers once filled the 

 wide valleys from bluff to bluff. The writers apparently have never 

 tried to calculate how much water would be needed to fill these 

 sloping valleys, and are unaware that a pluvial period with a rainfall 

 of double or treble the present amount would be quite insufficient, and 

 that at least one hundred times the present fall would in many cases 

 be needed. The existence of these sheets of gravel, and the constant 

 occurrence of deeply excavated valleys in regions where no streams 

 now exist, constituted the evidence on which was founded the well- 

 known Pluvial Theory. 



Let us, however, try to put on one side all preconceived views 

 and examine afresh the evidence. We will take first the fossils. 

 After comparing a number of collections from various old river- 

 deposits, and working out each one separately, it is possible to arrive 

 at a clearer idea of the climatic conditions that prevailed than could 

 be obtained by any mere examination of museum specimens or of lists 

 of species. Among the more striking results of the study is the 



