I saw an extract that valleys of Switzerland were wholly due to glaciers.

He cannot have reflected on valleys in tropical countries.

The grandest valleys I ever saw were in Tahiti.

Again, if I understand, he supposes that glaciers wear down whole mountain ranges; thus lower their height, decrease the temperature, and decrease the glaciers themselves.

Does he suppose the whole of Scotland thus worn down?
Surely he must forget oscillation of level would be more potent one way or another during such enormous lapses of time.