REPORT OF THE DIRECTOR, 1922 It3 
that it had been transformed by supergene (weathering) agencies 
from a sulphide lead-zinc bearing ore as the original; that this 
original ore had been itself introduced into the limestone rock sub- 
sequent to its own formation and deformation; and that the prin- 
cipal present ore values did not occupy the place of the original sul- 
phide minerals but, instead, had themselves replaced portions of the 
limestone, and that there was some selective distribution. The ore 
was strictly a superficial or near-surface deposit and had no chance 
of continuing in depth below the oxidized zone. 
2 An old silver mine in Mexico, said to have been abandoned 
after reaching a certain level, not because of any fault with the ore, 
had been examined by competent mining engineers and sampled. 
Because, however, of difficult conditions at that time in Mexico, all 
of their shipped samples had been lost or destroyed, only a handful 
of small chips remaining, which one of the engineers had with some 
forethought carried out in his pocket. 
The question they wished to ask was a very direct one, that 1s, 
has the mine been worked out, or is there real evidence to show 
that the ore still continues at the bottom much like that at higher 
levels, so that additional work might hope to find workable values 
at still greater depth? 
It is a perfectly good petrographic problem. The conditions are 
unusual and discouraging ; but it can be solved by those methods or 
else it cannot be solved at all. One must have a working knowledge 
of the origin of ores of that kind, and of the principles of alteration 
and secondary enrichment with their influence on distribution. One 
must be told about the relative distribution of these samples taken 
from the old mine and something of the structural facts; then, if he 
can read the criteria and interpret the evidence, he can venture an 
opinion based on real data, even in such an extreme case as this. 
3 Several years ago the author published a short description of 
the “bluestone”? of New York, making the study chiefly one of 
composition and micro-structural condition. It was not by any 
means well done in one respect at least, its interpretation. It was 
noticed that most of the original grains are now made up of aggre- 
gates, but it was then thought that these aggregates were secondary, 
resulting from the alteration of original, simple mineral grains. 
This interpretation is certainly wrong. These grains were originally 
aggregates and are not at all due to subsequent alteration. It 
is certain, also, that some of these grains, perhaps nearly all of them, 
are disintegration products from older formations which were them- 
selves fine-grained aggregates in makeup, such as slates or argillites. 
