384 Murchison’s Siluria. 
that a very considerable amount of the precious metal may not 
be obtained from workings in the veins themselves, if properly 
and economically managed. 
ithin the last few years, the ratio of the production of gold 
to silver has undergone a great change. Early in the present 
century it was by weight nearly as 1 of gold to 43 of silver; 
in 1845 when the Siberian washings were most productive, the 
ratio was 1:17; in 1852, when California and Australia were 
yielding most largely, it was as 1:4. It has been generally 
admitted by writers on political economy, that so great a change 
as this in the relative amount of the two metals brought into use 
must have an effect on their relative values, and that silver must 
gradually rise to something approaching the value which it former- 
ly had as compared with gold. Murchison however has the extra- 
ordinary idea that “ Providence seems to have originally adjusted 
the relative value of these two precious metals and that their rela- 
tions having remained the same for ages, will long survive all the- 
ories.”” He seems to forget, that the opening of the silver mines 
to South America in the sixteenth century so increased the produce 
of that metal as to depress its value, as compared with gold, from 
a ratio of about one to ten, which it had maintained for a long 
riod, to one to fifteen. Since the earliest historical times the 
relative value of the two noble metals has always been more or 
less fluctuating, and, being dependent on the varying influences 
of demand and supply, it will no doubt always continue $0 ; 
although, except under extraordinary circumstances, such as the 
discovery of the California gold-fields may be conceived to be, 
usually vibrating within narrow limits. 
he last chapter of the Siluria is chiefly devoted to a general 
view of the succession of life from a beginning, as based on 
itive observation, carefully distinguishing absolute geological re- 
sults from mere theoretical speculations on what may hereafter 
be found. The idea of a progressive development in animal an 
vegetable life, which was for a time so stoutly battled against, 
seems to be more generally recognised. It is now seen that in the 
general anxiety which once prevailed among geologists to bring 
down the higher types of organized existence into the lowest 
strata, a good many mistakes were made, which more careful ob- 
servations have set right. Thus the famous tracks in the Pots- 
dam sandstone of Canada, which were once so confidently pro- 
nounced to belong to Chelonians, are now regarded as Crustacean. 
At one time we were led to believe that fishes made their appeat- 
ance as low down in the series as the Lower Silurian, and this 
was strongly insisted on as opposed to the theory of progression. 
It now seems, however, to be almost certain from the compariso! 
of observations made-all over the world, that it is not until we 
rise to the uppermost beds of the Silurian System that we find 
