44 HAMPSTEAD HILL. 



both sub-tropIcal and temperate marine creatures might find 

 waters of the temperature they required. 



The warmer cHmate of this area at the period of the forma- 

 tion of the London Clay has formed the subject of much 

 discussion, and various ways of accounting for it have been 

 suggested. Some suppose a change in the position of the 

 earth's axis, some a greater heat of the sun ; but it should be 

 remembered that changes of geographical outlines of land and 

 sea produced by those slow and long-continued upward and 

 downward movements of the earth's surface, which, in some 

 region or another, have always been going on, will account for 

 much climatal alteration. By supposing a sea closed to all 

 northerly currents of cold water, and open to the south-west 

 by which it would receive the full benefit of those warm flows 

 from the mid-Atlantic which even now raise the temperature 

 of the water on our southern and western coasts, we may easily 

 account for a very considerable increase of temperature at a 

 particular epoch. 



