_ 
32 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
dip under older rocks, still further east appear, therefore, to favor the 
notion that the paleozoic rocks of the Appalachian region and the 
eastern part of the Mississippi basin were derived from the erosion 
of highlands formerly existing east of the Appalachian chain, now, 
perhaps, submerged in the Atlantic ocean. The downthrow of a 
fault, if formed in the manner supposed in the region under con- 
sideration, would accordingly be on its western side, as suggested 
above. 
The third communication was by Mr. S. F. Emmons on 
, ORE DEPOSITION BY REPLACEMENT. 
[ Abstract. ] 
After a few introductory remarks upon the relatively unsatis- 
factory condition of that branch of geology which treats of ore de- 
posits, considering the early date at which it was taken up, the 
speaker briefly reviews the existing theories and classifications, and 
shows that they are mainly based on the idea that each ore deposit 
is the filling of some pre-existing cavity or opening in the rock in 
which it is now found ; that so-called fissure veins, for instance, were 
once actually open cracks, and that irregular deposits in limestone 
have been made by the filling up of open caves, such as so fre- 
quently occur in these rocks. The result of his studies of the so- 
called “carbonate deposits” of Leadville, Colorado, has been to 
show that they are not the filling up of pre-existing cavities; the 
caves there have been formed since the ore was deposited, as is 
proved by their crossing indiscriminately ore bodies and limestone. 
‘They belong to a class of deposits for which he proposes the name 
metamorphic deposits, or those which have been formed by a meta- 
somatic interchange between the vein and original rock material. 
In Leadville the principal deposits are an actual replacement of 
the limestone itself at or near the contact of this stratum with an 
overlying sheet of porphyry. This replacement action has in places 
proceeded so far that the entire stratum of ore-bearing limestone or 
dolomite, originally 150 to 200 feet thick, has been cHanged into 
vein material, which consists of silica and metallic minerals. This 
vein material was brought in solution by percolating waters, which 
had taken it up during their circulation through the adjoining and 
generally overlying eruptive rocks. A more detailed description 
of the phenomena of these deposits will be found in his paper en- 
