306 Proceedings of the Royal Physical Society. 
Time in Years. 
Brought foward, . 361,735,000 
Time required for the Formation of 2500 feet of Carboniferous ; 
Limestone, : : : . 62,500,000 
Time represented by the Coie Dba Tanieavaiee . 125,000,000 
Duration of the Period represented by the Salopian (Silurian) 
Rocks, F 6 2 - 56,000,000 
Chronological Value of the Tieonforsaity at the Base of 
the above, . ; 12,000,000 
Time required for the Chee ae of the Tiptee Ondavician 
Rocks, ; : . . 7 : ° , : - 12,000,000 
Lower Ordovician Time, . . é F - : . 33,000,000 
Upper and Middle Cambrian Ported? ; é : : . 36,000,000 
Lower Cambrian Period, . ; é ‘ : : : . 6,000,000 
Total since the Commencement of the Cambrian Period, . 704,235,000 
Concluding Remarks—Seven hundred millions of years 
back from the present to the commencement of the Cambrian 
Period may seem to many persons far too extravagant an 
estimate to be for a moment seriously considered. Every 
one of the data upon which this estimate is founded will 
probably be regarded as absolutely untrustworthy, on the 
ground that they are avowedly based almost as much upon 
speculation as upon fact, and also upon the ground that 
I myself do not put these conclusions forward as a final 
answer to a very difficult question, and one which probably 
never will be answered with more than a very rough 
approximation to the real truth. If the estimate prove 
wrong in any particular, then the very rectification of the 
error in question will bring us a step nearer to the truth, 
and no one will rejoice more than I shall at finding what I 
feel is little else than a speculation, based as yet upon 
imperfectly-known data, replaced by facts and inferences 
therefrom which are of a more trustworthy character. 
Hypotheses have their use in Geology as much as in other 
branches of Science. 
For my own part, I am quite prepared to find that, as our 
knowledge of Geology and Biology advances, we shall feel it 
necessary to extend rather than to abbreviate the estimate I 
have put forward. 
Seven hundred millions of years carry us back to the 
