324 ^/'^ America?i Geologist. May, 1900 
meeting of the American Institute of Mining Engineers, we have the 
subject of ore origin approached strictly from the direction of modern 
geology. Altogether the treatment is noteworthy. It may be re- 
garded as marking a new era in ore study. However, few persons 
who are not expert petrologists in the most modern sense of the 
word will fully appreciate the profound significance of this author's 
statements. It shows more conclusively than ever before that if we 
are to make rapid progress in the study of ore genesis, it must be 
largely along geological lines. Geological relationships, geological 
structure, and geological processes come in for first consideration. 
While the present paper is an adaptation to the ore deposits, of a 
more elaborate treatise on general rock metamorphism, now nearly 
ready for publication, its presentation at this time is timely. The 
general subdivisions of the subject are self-explanatory. The litho- 
sphere is divided into zones of fracture, combined fracture and 
fiowage, and flowage. The water content and the openings in the 
rocks are next taken up. In a third section are considered the 
physico-chemical principles controlling the work of underground 
waters; and in a fourth, the general application of these principles to 
ore deposits, the work of ascending water, of descending water, and 
the various reactions are clearly set forth. 
An almost startling inference is it that ore bodies are essentially sur- 
face deposits; that is, they are mainly confined to the brittle cuticle 
of the globe, that zone near the surface which is commonly called by 
m.odern geologists the zone of fracture. Few of us are fully prepared 
to accept the proposition without some hesitation. Yet a little re- 
flection will show that it could hardly be otherwise. The phenomena 
connected with ore deposits are merely special cases of the more 
general problem with which geologists have to deal. 
The consideration of the upper limit of groundwater is particularly 
opportune. Regarding its relations to the position and character of 
ore bodies it removes, at once, many obstacles that have long stood 
immovable in the path of satisfactory explanation of apparently 
anomolous phenomena. Any change of position of this groundwater 
level necessarily produces important changes in the mineralogical 
nature of the ores. Yet orogenic movements are known to take 
place in many cases much more rapidly than the ores are altered by 
weathering influences. Consequently we often find a more marked 
discrepency between the groundwater line and the local character of 
the ores than we should expect. In the broader field of general rock 
alteration we assume the truth of the observation made by Wads- 
worth and by Judd, that all such changes are from a less stable to a 
more stable condition. But this does not express the full significance 
of the phenomenon. The conditions themselves are continually 
changing. The process is essentially continuous, sometimes in one 
direction, sometimes in directly the opposite. It does not seem to 
be making the statement to strong when it is said that the changes 
