Economic Geological Deposits. — Crosby. 251 
knowledge of genetic processes. In this connection we may 
properly draw a sharp distinction between original and sub- 
sequent structures, the former alone having classificatory value. 
The validity of this distinction is apparent on referring to the 
biologic sciences, since, for example, the accidental loss of one 
or more of its rays by a starfish does not affect its position 
in the classification. Of course, changes in geologic deposits 
amounting to complete metamorphism. to the transformation 
of one type into another, are truly original and genetic, and 
not in any sense accidental, with reference to the new type. 
Thus superficial oxidation may be neglected in the classifica- 
tion of veins, but it is highly important as defining one type 
of residual deposits. Distinctions based directly upon com- 
position should be subordinate, in the main, to original struc- 
tural distinctions, and hence can be generally recognized only 
in a more complete and detailed classification than is possible 
in the present state of our knowledge. Even a classification 
based upon origin and original structure, such as is attempted 
here, is necessarily in advance of our knowledge, in the sense 
that future investigations, in many cases, must decide to which 
sections particular deposits should be referred. 
Many classifications of ore deposits have been published; 
and the more important of these have been recently summar- 
ized and compared by professor Kemp, in the introduction to 
his valuable work on the ore deposits of the United States. 
But, since metalliferous minerals or ores constitute simply one 
limited division of the whole field of economic geology, it is 
apparent at a glance, after what precedes, that these classifi- 
cations must be, without exception, seriously lacking in sci- 
entific and educational value. They may be said, perhaps, to 
meet, in some degree, the needs of the metallurgist; and in a 
measure, also, those of the miner. But coal and many other 
non-metalliferous minerals are extensively mined and occur 
in characteristic deposits, with the forms and modes of origin 
of which the miner should be acquainted. 
The distinction of rocks (igneous and sedimentary forma- 
tions) and mineral deposits (veins, impregnations, etc.), or, 
as it is commonly stated, original and secondary deposits, or 
unconcentrated and concentrated mineral deposits, is deemed 
of great importance by some authorities, and notably, among 
