342 THE CAUSES OF THE ‘ XI 
the real limitation of their inquiries, and to 
forget the extreme imperfection of what is really 
known. : Geologists have imagined that they could 
tell us what was going on at all parts of the 
earth's surface during a given epoch; they have 
talked of this deposit being contemporaneous with — 
that deposit, until, from our little local histories of 
the changes at limited spots of the earth’s surface, 
they have constructed a universal history of the 
globe as full of wonders and portents as any other 
story of antiquity. 
But what does this attempt to construct a 
universal history of the globe imply? It implies 
that we shall not only have a precise knowledge of 
the events which have occurred at any particular 
point, but that we shall be able to say what events, 
at any one spot, took place at the same time with 
those at other spots. 
Let us see how far that is in the nature of 
things practicable. Suppose that here I make 
a section of the Lake of Killarney, and here the 
section of another lake—that of Loch Lomond 
in Scotland for instance. The rivers that flow 
into them are constantly carrying down deposits — 
of mud, and beds, or strata, are being as constantly — 
formed, one above the other, at the bottom of © 
those lakes. Now, there is not a shadow of doubt — 
that in these two lakes the lower beds are all 
older than the upper—there is no doubt about 
that; but what does ¢his tell us about the age of 
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