250 Tht American Geologist. April, 1891 
feels a vital interest, might be disposed to criticise such a clas- 
sification as unnecessarily complicated and as implying a 
more extended knowledge of general geology than he usually 
possesses: but for the teacher and the student of economic 
geology and all who care for broad views over this field, it 
would possess obvious advantages. 
It is generally conceded that, whatever the limits of the 
classification, its highest value, both as a means of education 
and as an aid in the discovery and exploitation of deposits, 
demands that it should be essentially geological, and not be 
based, at least in its main lines, upon the qualities or uses of 
the materials. Now, geolog}' is, first of all. a dynamical sci- 
ence; and. as professor Kemp has shown,* there are cogent 
reasons for basing a classification of economic deposits upon 
origin instead of structure. The genetic principle, so univer- 
sally recognized as the basis or key-note of biologic classifi- 
cations, is slowly but surely making its way in the domain of 
geology. ( 'lassification bj r structure, of which for in is but the 
external expression, is less difficult, and hence has generally 
prevailed in the early days of each science. The main reason 
why the recognition of the genetic principle lias made, rela- 
tively, so little progress in geolog}', is that the processes, being 
to a large degree chemical, are often exceedingly obscure; 
and the correct interpretation of the observed facts of com- 
position and structure is hence peculiarly difficult. Our 
knowledge is, in consequence, decidedly insufficient, in certain 
directions, for the detailed application of this principle of 
classification. It is, for example, the general belief of geolo- 
gists that mineral veins have been formed chemically, that is. 
by the deposition of minerals from solution; but further than 
that, as a rule, we cannot safel}^ go: for the nature of the 
chemical process in particular cases is still a matter of spec- 
ulation rather than of positive knowledge. The recent litera- 
ture of the science shows, however, that light is breaking 
upon this problem in various directions, and we may hope for 
a better state of things in the near future. 
It appears, then, that while differences of structure and 
composition must always be recognized in our classifications, 
tiny will become relatively less important with increasing 
*Ore deposits of the United States, p. 42. 
