14 The American Geologist. January, 1905 
of mining geology. The mechanism of the secondary en- 
richment of ores, particularly those of copper, detected by 
Mr. S. F. Emmons and enlarged upon by Mr. W. H. Weed, is 
being studied experimentally in the laboratories of the 
U. S. Geological Survey. A feature deserving careful ex- 
perimental study is the osmotic separation of ores from their 
solutions by the wall rock. Many minutiae of occurrence 
suggested that the walls of veins often act as a species of 
diaphragm or molecular filter and have a dialytic action on 
the ore solutions.* The origin of the ores themselves is still 
very obscure and will hardly be elucidated until more is 
known of the earth's interior. Sometimes they seem to be 
derived from adjacent rocks ; in other cases conditions sug- 
gest that the rocks and the veins derive their metallic con- 
tent from a common deep-seated source. Here, as in several 
other connections, professor Suess's theory of "juvenile wa- 
ters" is very suggestive. It is held that many of the great 
iron deposits are due to magmatic separation. Depositions 
of lead ores by replacement of calcite is a known process, 
but takes place under unknown conditions. In some cases 
replacement of rock by ores appears to me to be alleged 
without sufficient proof. Pseudomorphosis is the only ade- 
quate test of replacement. 
Erosion appears to be a subject which is capable of more 
•exact treatment than it has received. Weathering and 
abrasion proceed with a rapidity which increases with the 
surface exposed per unit of volume.* Hence these processes 
lead to minimum surfaces. Therefore also the mathematics 
of erosion is essentially identical with that of capillarity. 
Geological chmates are as interesting to astrophysicists 
as to meteorologists and geop'hysicists. Messrs. Langley 
and Abbot appear to have evidences of recent variations in 
solar emanation. If these have been considerable in the 
course of the period of historical geology, light should be 
thrown upon them by the paleontology of the tropics. Vari- 
ations in the composition of the atmosphere must have been 
very influential in determining both the mean temperature of 
the earth's surface and the distribution of temperature; but 
so also is the distribution of water. No theory of the glacial 
• "Min. Resources of the U. S. for 1892," p. 156. 
* U. S. Geol. Sicney. Mon. XIII. 18S8, p. 68. 
