430 
THE GEOLOGIST. 
also have occiin'ed, entii-ely altering the dissolving and precipitating 
power of the circulating fluid, and consequently giving us, in the 
newer fissures, a set of minerals, which although they existed in 
circulation in the older fissures, could not be precipitated in con- 
sequence of the higher tempcratui'c then existing keeping them in 
solution ; while, sirailai-ly, the newer fissures could not contain many 
of the minerals found in the older ones, inasmuch as the temperature 
had now so fallen as to render the circulating fluid incapable of 
holding them in solution. 
3. The analogies found to exist between the relative age and general 
characteristics of veins in countries widely apart, which gave rise to 
the theoiy of " formations," are accounted for by assuming them to 
be the " everywhere tolerably analogous consequences of local 
eruption, which may have been veiy far separated from each other 
by time." The supposed synchronous "formations" merely "re- 
presenting the same stadium of local activity," each stadium 
being assumed to produce results tolerably analogous, however 
widely removed in time and distance each isolated process may 
have occurred. 
4. Cotta also finds in the the structure of the Freiberg veins a proof 
of the general tnith of his hypothesis. The older veins he finds to 
be generally massive, while the newer have a banded, or layer -like 
form ; these structures, in both cases, being such as might a prion 
have been expected. 
5. With regard to the infiltrative origin of veins, it is admitted to 
be possible, although not probable, for all ore-veins ; as to the newer 
and banded veins such an origin seems to be accepted. Tin-ore is 
considered as not being found in purely infiltrative veins, although 
cii'cumstances are referred to which seem to indicate that there are 
cases where that ore cannot have otherwise originated. 
X. General remarlcs. — Although this hypothesis afibrds a general 
solution of many obscui'ities, it is not pretended that it is itself with- 
out difficulties : the whole question of metalliferous deposits is far 
too involved with various complicated geological phenomena to be so 
lightly dealt with. The hypothesis is at best a suggestion for con- 
sideration ; but it has this advantage, that it embodies in a defiinite 
form a set of facts, with which it harmonizes admirably. The 
