254 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
eral observations in Santiago province show. The structure in the 
immediate vicinity of the Boston and Ponupo mines is quite dif- 
ferent. These lie well within the great syncline, where the strata 
have been thrown into minor folds, and it is observed that the ore 
deposits in both places occupy the central or axial portion of articlines 
or arches of the strata. The Ysabellita mine is less than a mile from 
the Boston, and appears to be located upon the same arch, but the 
structural relation between this fold and the one at the Ponupo mine 
is not known. Though the altitude of the strata is different, the 
relations of the ore, jasper, and country rock are exactly the same as at 
Cristo, and the deposits have been the result of metasomatic replace- 
ment of calcareous strata by manganiferous minerals and jasper. In 
the case of the Boston, Ysabellita, and Ponupo mines, and probably also 
at San Nicolas, it may be argued that the hot waters to which the 
-replacement is attributed ascended through rissures locally developed 
along the axis of the folds rather than through the strata, as has been 
suggested for the occurrences in the vicinity of Cristo, where the 
stratified formations are standing on end. This would account for the 
local character of the deposits along the folds, as well as the presence 
of undissolved shells and the absence of both jasper and ore in the 
beds of green sand as they rest on the flanks of the arch at the Boston 
mines. 
In both the Boston and Ysabellita mines the amount of jasper is 
large, and it occurs in large masses, around which the richest ore is 
found, with deposits in which the ore is mixed with rock, dissemi- 
nated locally in the portions of the decomposed greensand or glau- 
conitic rock adjacent to the jasper. Sometimes the ore is found to 
entirely surround the masses of siliceous rock along its contact with 
the country rock. An illustration of this is seen in the Boston mine, 
where a large block of jasper has been worked about on all sides and 
a large amount of ore extracted. Another is seen in the workings of 
the Ysabellita mine. Next to the large mass of jasper a bed of 
loose, sandy material, containing oxide of manganese in the form of 
small nodules arranged along the planes of stratification, extends to a 
distance of not less than 25 feet from the jasper, and the thickness 
of the ore-bearing bed is not less than 20 feet as exposed. 
In this vicinity there are no less than six distinct outcrops of jasper 
in large masses within a radius of about 150 feet, but only one has 
been sufficiently developed to prove the presence of large quantities 
of ore. 
At the Ponupo mine the conditions are quite similar to those at the 
two mines just mentioned. The deposit occupies the center of an 
anticlinal fold, and there are large amounts of jasper, with high-grade 
ore occurring in contact with it, and ores of lower grade, because 
mixed with decomposed rock lying adjacent to it. Here the ore 
extends up to the horizon of the foraminiferal limestone, which it 
