Economic (jcoloijicdl Deposits. — Crosby. 257 
and non-nietasomatic ; and substitution deposits, or complete 
metasomatic replacements. 
1. Veins, or deposits in pre-existing- cavities. — The line of 
demarcation between impregnations and substitution depos- 
its is, in the nature of the case, less sharply defined than that 
between either of these classes and veins, since metasomatic 
impregnations must grade into complete replacements. A 
pre-existing cavity or free, continuous space is regarded as 
essential to the formation of a vein: and all deposits in such 
cavities, without regard to the forms or modes of origin of the 
cavities, are here classed and described as veins. It should 
be noted, however, that the cavities are usually, if not always. 
secondary features of the wall-rock, the product in general of 
displacement, plication, shrinkage, or solution. The only im- 
portant instance, so far as I know, of cavities contemporane- 
ous with the walls, which might conceivably become the seats 
of veins, is afforded by the lava tunnels which area somewhat 
important feature of the Hawaiian and some other volcanoes. 
It seems extremely probable that cavities formed in this way 
have in some instances been filled by deposition from solu- 
tion; but so far as I am aware this mode of occurrence has 
not been demonstrated, at least not for any economic depos- 
its', although it is well known that the chimney form, which 
this origin would require, has been clearly recognized in cer- 
tain ore deposits. The discovery of an undoubted instance of 
such a volcanic vein of economic interest would necessitate a 
fundamental distinction between veins tilling original and 
those filling-secondary cavities, all the types of veins recog- 
nized in this paper belonging to the second class. 
True veins are, of course, always to be reckoned as struc- 
tural rather than textural features of the formations which 
they traverse ; and interstitial deposits, due to the filling of 
steam holes and other forms of interstices, are types of im- 
pregnations and not of veins. It may he noted, however, that 
the larger steam holes pass gradually into irregular cavities 
of considerable size, which are also often filled with segrega- 
tions of chalcedony, etc., and clearly, point to the possibility 
of veins in lava tunnels. 
The following classification of veins formed in secondary 
cavities is based upon the modes of origin, and only incideii- 
