260 Proceedings of the Royal Physical Society. 
the conception of the equally vast intervals of time with 
which it is his own special province to deal.} 
“Vague, indefinite, but unquestionably vast beyond con- 
ception” is a phrase that must recur to the minds of most 
geologists when referring to the subject of Geological Time. 
Yet, indefinite though it must long remain, it seems to me 
that almost every attempt that has been made at reasoning 
out an answer to the question, “ What is meant by Geological 
Time ?” has contributed something that has helped to make 
our ideas upon the subject more and more definite. Specula- 
tion is not without its uses in science; and even when it 
may be based more or less upon error, the correction of that 
error may at least help to guide others into the track which, 
sooner or later, will lead them up to the truth. 
In dealing with this subject of the Age of the Earth, the 
fundamental idea which a geologist steadily keeps in mind 
is that all the changes, physical and biological, which the 
records of the rocks inform us have taken place upon the 
Earth in the Past, can only be understood and properly inter- 
preted by referenee to changes of the same nature which 
are known to be in progress during the Present. This, of 
course, does not imply absolute uniformitarianism (as this is 
commonly understood), but it allows for catastrophism in 
certain exceptional cases, along with normal uniformity of 
action in the rest. The geologist obtains abundant confirma- 
tion of the justice of this view in the fact that, throughout 
the whole series of rocks, and even throughout those strata 
which are older than the most ancient yet known to contain 
records of life, the manifestations of Nature’s forces show no 
sign of any action different, in either degree or kind, from those 
which are known to be at work at the present day. Rain 
fell in those early days much as it falls now. Wind blew 
then with apparently no greater force than it does to-day. 
The tides and currents of those early periods appear to have 
obeyed exactly the same laws that they do now, and in no 
1 The astronomical models in the Museum of Practical Geology, and the 
more extended set placed in the Gallery of Scottish Geology and Mineralogy 
in the Edinburgh Museum of Science and Art, were set out with the express 
object here referred to. 
