ORE DEPOSIT THEORIES. 207 
have taken up substances such as hydric and alkaline sul- 
phides into solution, rendering them reducing in their 
action. It is to the action of these deep underground circu- 
lating waters leaching out the minute metallic ‘contents of 
the rocks and depositing them in fissures and cavities, or in 
the spaces vacated by more or less soluble rocks, that the 
meteoric school attribute the origin of the majority of ore 
deposits. 
That the common eruptive and many of the sedimentary 
rocks forming the crust of the earth are competent to supply 
the material for ore deposits, provided the means for con- 
centration exist, is now admitted by the majority of 
geologists. The facts have been fully demonstrated by 
exhaustive analyses carried out by such investigators as 
Sandberger, Stelzner, Robertson, Don, and many others. 
The content is usually exceedingly small, going down as low 
as hundredths or thousandths of 1 per cent., or even lower 
than this, but still it exists. It has been further demon- 
strated that the metallic contents of igneous rocks are much 
greater than those of sedimentary rocks. Speaking very 
generally, it may be said that the metallic contents of 
igneous rocks are greater by about one decimal place than 
the contents of sedimentary rocks; by that amount, then, 
are igneous rocks the more favourable sources of the metallic 
contents of ore deposits. 
Given, therefore, an agent capable of effecting the con- 
centration of these minute metallic constituents distributed 
through the rocks, and we have evidently the materials 
for a working theory of the origin of ore deposits. 
We have now, therefore, to enquire into the competence 
of this deep undergound water circulation to bring about 
such a concentration. The subject is a very large one, and 
cannot be fully discussed in the present paper. 
The points which most need elucidation are the fol- 
lowing :— 
1. The porosity of rocks. 
2. The motive forces producing circulation. 
3. The nature and direction of the underground 
flowage. 
The Porosity of Rocks. 
IT am not aware that any investigations have been under- 
taken with the object of definitely ascertaining the size and 
distribution of the openings in various rocks. Such an in- 
vestigation would no doubt be extremely laborious, and it 
is doubtful if results could be obtained which would have 
much scientific value. Without any definite data, it is 
