898 THE BEAUTIES OF NATURE CHAP, 
and striking, have had, after all, comparatively 
little part in the result. 
The mouths of rivers fall into two princi- 
pal classes. If we look at any map we cannot 
but be struck by the fact that some rivers 
terminate in a delta, some in an estuary. The 
Thames, for instance, ends in a noble estuary, 
to which London owes much of its wealth 
and power. It is obvious that the Thames 
could not have excavated this estuary while 
the coast was at its present level. But we 
know that formerly the land stood higher, 
that the German Ocean was once dry land, 
and the Thames, after joining the Rhine, ran 
northwards, and fell eventually into the Arctic _ 
Ocean. The estuary of the Thames, then, 
dates back to a period when the south-east of 
England stood at a higher level than the 
present, and even now the ancient course of 
the river can be traced by soundings under 
what is now sea. The sites of present deltas, 
say of the Nile, were also once under water, 
and have been gradually reclaimed by the 
deposits of the river. 
It would indeed be a great mistake to 
