820 
The American Geologist. 
May, 1896 
Origin and mode of formation of the reefs. Answers to 
some objections. 
The origin and the mode of formation of the gold deposits 
by the circulation of mineral waters in the porous strata seems 
to explain all the facts observed. Mr. de Launay objects to 
what he calls this very seductive theory of auriferous and 
pyritiferous impregnation of the conglomerate on the ground 
that it does not satisfactorily account for beds of pebbles of 
similar size and identically the same structure even over an 
extent of several meters, of which some are gold-bearing and 
others barren ; and thinks it is absolutely incompatible with 
the constant presence of rolled pyrites. 
This objection is answered by assuming variations in the 
composition of the mineralizing waters. Coming from the 
deep regions of the earth’s crust they followed, as already de¬ 
scribed, the porous strata. Let us suppose a stratum of con¬ 
glomerate traversed by mineral waters of variable composition. 
Primarily siliceous, they subsequently brought pure pyrite and 
then gold with the pyrite. Such was the origin of a reef. 
The siliceous waters would have commenced the work of ce¬ 
menting the quartz pebbles, and would have formed around 
certain centers in the middle of the bed more or less irregular 
areas of agglomerated conglomerate, which would become im¬ 
pervious as soon as the cavities were filled with silica. 
If it be supposed that at a given period the mineral waters 
then brought up sulphide of iron, it would form around these 
sterile areas in the shape of specks in the strata of those 
zones which contain iron pyrites. The latter would them¬ 
selves be surrounded by other zones of auriferous pyrites 
when the epoch of gold deposition began. The cavities once 
filled, the reef is formed. Thus the juxtaposition of rich and 
barren zones of auriferous pyrite is easily understood. It is 
also evident, up to a certain point, that the average gold con¬ 
tent of the stratum thus formed would be comparatively con¬ 
stant in a homogeneous area. 
The pyri tiferous zones parallel or oblique with the stratifi¬ 
cation, the influence of non-porous schists on the walls, on 
the richness and continuity of the reef, the influence of large 
pebbles, etc., are all easily explained. Some entire strata have 
been cemented by silica alone, others by pure iron pyrites, and 
hence there are sterile beds alongside of rich ones. 
