This innovative book uses the story of how a modern science achieved its present shape and focus to introduce the question of the nature of scientific change and its philosophical analysis. The "modern revolution in geology" of the 1960s and 1970s saw the triumph of the global theory of plate tectonics, and decisive turning point in fifty years' controversy and competition first sparked in 1912 by Wegener's proposal of continenta drift. Here, Professor Le Grand interweaves a history of this episode of scientific change with reflective discussions of its historical, philosophical, and social circumstances, and of the development of science more generally. The approach of the book is exploratory; the reader is encouraged to be an active participant--to use the historical narrative to understand and criticize some of the more recent, influential ideas about science and scientists, to draw conclusions, and especially to pose questions about how and why changes occur in scientific knowledge and practice.