Complex sentence: Before the concept of plate tectonics, global cooling was a reference to a geophysical theory, The Contracting Earth by James Dwight Dana, that the Earth had been in a molten state, and features such as mountains formed as it cooled and shrank.
Split into simpler sentences: Before the concept of plate tectonics, global cooling was a reference to a geophysical theory described in The Contracting Earth by James Dwight Dana. It suggested that the Earth had been in a molten state, and features such as mountains formed as it cooled and shrank.