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be in use among petrographers for many more; and that, by and by, 
a whole new system of nomenclature will be worked out, designed 
to emphazie these principles. 
Examples 
1 Geologists speak of the residue left by the evaporation of a 
solution as a precipitate; but there is no name for the product of the 
reaction between two mixing solutions, such as seems to have pro- 
duced some of our ores. Toa petrographer or a chemist the latter 
is the better precipitate, but he would like at least to distinguish 
between them. The genetic idea is carried better if the product of 
evaporation were called an evaporate, as Doctor Grabau does in his 
work on Salt Deposits, and the product of reaction by mixing were 
called a reactionate. Perhaps one could write them evaporite and 
reactionite. 
2 These are still different from the product of simple reduction 
of temperature or pressure in an ascending, saturated solution, for 
which there is noname. Is it to bea saturite? And for the product 
of reaction between a solution and a rock wall, resulting in precipi- 
tation from the solution, there is also no name. 
3 These are all very different genetically from the product of 
reaction between solution and rock wall or host when the reaction 
causes reorganization of portions of the rock itself, taking some 
thing out of the solution to add to its own constituents. It is a 
form of reactionite also, but very different indeed from either of 
the others. There must be a whole family of reactionites differing 
in various, quite fundamental ways one from another, and altogether 
producing a most interesting and important lot of petrographic 
materials encountered in many practical problems. 
If one has worked little with ores and their relations, these needs 
would not necessarily have been felt; but if one has followed applied 
petrology with mining engineers he will have discovered that there 
is no name for a very great many of his perfectly typical things. 
One does not need to invade that field, however, to feel this poverty 
of nomenclature. The fact 1s that geology as related to petrogenesis 
has outstripped petrography as a separate descriptive branch, and 
there are now many perfectly clear concepts of processes and prod- 
ucts for which there are no petrographic terms. 
4 Sometimes one substance completely replaces another, or cer- 
tain ones completely replace others, producing an entirely new com- 
position. There must be a whole family of these types. I suppose 
