314 Proceedings of the Royal Physical Society. 
would only restrict ourselves to facts, and be governed by 
scientific methods. Under this conviction, I offer the fol- 
lowing remarks. If to some they shall seem inconclusive ; to 
myself, at least, they appear as explicit as the facts will 
warrant, and the topic throughout has been dealt with in 
the same manner as would any other that comes legitimately 
within the scope of geological inquiry. 
Unlike the periods of human history, those of Geology 
have no definite expression in years and centuries. We 
speak of eras and epochs, of cycles and systems, but these 
are merely relative terms. They have no definite duration ; 
the one merely precedes the other, and the larger may in- 
clude many recurrences of the lesser within its limits. In 
speaking of geological time this is all that is signified ; in 
fixing the dates of geological events this is all that can be 
fairly asserted. The Primary merely precedes the Secondary, 
the Secondary the Tertiary, and the Tertiary the events of 
the current epoch. We may subdivide these greater stages 
into narrower limits, and talk of Laurentian, Cambrian, 
Silurian, Devonian, Carboniferous, Permian, Triassic, Oolitic, 
Cretaceous, Tertiary, and Eecent rock-systems, and this is 
no doubt restricting events to more precise bounds ; but it 
gives no definite idea of duration, nor tells us how long the 
Chalk preceded the Tertiary, or the Tertiary the occurrences 
of the existing epoch. We can judge from its thickness, 
and the nature of its rocks and fossils, that one system took 
much longer time to accumulate than another, but we can- 
not venture, by any known method of computation, to say 
how long in years. All that we have to do with is relative 
time ; and even in dealing with the current epoch, should 
we assert that certain events took place more than six i 
thousand or eight thousand years ago, we are simply assert- 
ing a provisional opinion, and not maintaining a belief hke 
that founded upon the written record of human history. j 
The geological history is thus relative and not absolute, j 
and when we arrange it, as in the subjoined tabulation, ( 
into Primary, Secondary, Tertiary, and Quaternary, we are 
merely asserting a certain order of succession, and this suc- 
cession not always clearly defined over certain areas. In- 
