will be gathered that these illustrations are fitly included
among pages which owe so much to Alpine inspiration. They
illustrate the

xv

subjects dealt with, and, it is to be hoped, they will in some
cases recall to the reader scenes which have in past times
influenced his thoughts in the same manner; scenes which in their
endless perspective seem to reduce to their proper insignificance
the lesser things of life.

My thanks are due to Mr. John Murray for kindly consenting to the
reissue of the essay on _The Birth-time of the World_ from the
pages of _Science Progress_; to Messrs. Constable & Co. for leave
to reprint _Pleochroic Haloes_ from _Bedrock_, and also to make some
extracts from _Radioactivity and Geology_; and to the Council of
the Royal Dublin Society for permission to republish certain
papers from the Proceedings of the Society.

_Iveagh Geological Laboratory, Trinity College, Dublin._

July, 1915.

xvi

THE BIRTH-TIME OF THE WORLD [1]

LONG ago Lucretius wrote: "For lack of power to solve the
question troubles the mind with doubts, whether there was ever a