316 Proceedings of the Royal Physical Society. 
rentian, or whether the Cambrian occupied a longer time in 
formation than the overlying Silurian. We may feel con- 
vinced, from the total thickness of a system, the alterna- 
tions of its strata, and the succession of its fossils, that it 
occupied a much longer time in formation than another sys- 
tem ; but this is not expressed in the above arrangement, 
which merely affirms a sequence from older to younger, and 
from the earliest ascertainable operations to those still taking 
place around us. 
Such is the chronology of Geology — a chronology to which 
investigators endeavour to conform the rock-formations of 
the globe ; and although the Chalk of one country, for ex- 
ample, may not have been exactly contemporaneous with 
the Chalk formation of another region, still we know that 
it stands intermediate between the Oolite and Tertiary, and 
can therefore assign to it a place relatively to these forma- 
tions. In some region yet unexplored a whole suite of 
strata may be discovered older than our Carboniferous, and 
yet younger than our Old Eed, and in such a case geologists 
would give the new formation a name, and place it as inter- 
mediate between these two systems. It would disturb no 
established order, but merely render more complete the 
sequence, like the interpolation of a hitherto unknown reign 
in the dynasties of human history. The geological record 
is thus a thing of mere sequence — an inconceivable amount 
of unexpressed time, during which certain events follow each 
other in definite order. How many ages have elapsed since 
the first deposition of the Laurentian strata we cannot tell ; 
how many centuries were spent in the formation of the 
Coal-measures of any locality, we can only, estimating from 
existing operations, offer the widest conjecture. Eut we 
can affirm with certainty, and this is a great point gained, 
that one rock-system is younger than another ; that these \ 
rock-systems follow in the order above given ; that accord- \ 
ing to our present knowledge invertebrate life preceded the j 
vertebrate ; that fishes preceded reptiles, reptiles birds, and | 
birds mammalia. We can also affirm, what it is the object 
of the following remarks to prove, that as there has been an 
ascent in time from lower to higher forms of life, so Man, 
