CHAPTER IV.

THEORY OF DENUDATION--CONCLUDED.

Glaciers--their Nature and Composition--their unceasing Motion --Powerful Agents of Denudation--Icebergs--their Number and Size--Erratic Blocks and loose Gravel spread out over Mountains, Plains, and Valleys, at the Bottom of the Sea --Characteristic Marks of moving Ice--Evidence of ancient Glacial Action--Illustrations from the Alps--from the Mountains of the Jura--Theory applied to Northern Europe--to Scotland, Wales, and Ireland--The Fact of Denudation established--Summary of the Evidence--This Fact the first Step in Geological Theory, 71