498 J. P. Kimball—Geological Relations and Genesis 
dykes, the identity of some of which seems to have been pre- 
served within the compass of their general overflow, or, as 
above distinguished, the dioryte mantle. The concentration 
of ferric and magnetic oxides upon such surfaces, presents 
outliers popularly regarded as outcrops of ore-bodies of great 
richness. A few blasts are generally sufficient to prove their 
superficial character. Cupreous stains of green carbonate from 
the epigenesis of a sulphide of copper, are found within the 
ore-bosses of coralline origin as well as in the interior of 
ferriferous parts of the dioryte. 
he copper deposits of the Sierra de Cobre west of the Bay 
of Santiago, exhibit to a remarkable degree still active metaso- 
matism of the diorytic porphyry in which they occur. 
only is the overflow of mine-water from the abandoned mines 
a present source of cement-copper, but exfoliations more or 
less cupreous are observed on all weathered surfaces of wall- 
rock left standing by the old English companies. Even 
individual fragments of dioryte detritus in the old burrows 
ave become completely coated with exfoliated mineral matter. 
Ferric, as well as miscellaneous cupreous products thus occur, 
along with a variety of silicates and other insoluble com- 
ounds. ven the old slags show zeolitic and other drusy 
segregations. Secular phenomena of this kind may be consid- 
ered as in part an effect of the humidity and high mean 
average temperature of the climate. 
Jnder the same favorable climatic conditions secular weath- 
ering, or metasomatism of eruptive basic rocks, has gone on to a 
remarkable degree throughout the whole region above briefly 
described. Permutations of this kind tend to produce from the 
eruptive, now represented by dioryte, a series of rocks resem- 
bling in lithological character metamorphic chloritic, garnetif- 
erous and ferriferous schists. The basic character of the pre- 
vailing admixtures of the latter, occurring in association with 
the ferric replacements of corallines, is their most obvious lith- 
ological point of difference from the prevailing type of Huro- 
nian specular schists, which as above remarked are essentially 
acid. Thus, they more closely resemble certain Laurentian 
magnetite-schists whose earthy admixtures are generally basic. 
Yet quarztiferous aggregates are not wanting among the great 
variety of ferriferous admixtures here referred to. 
e ferro-garnetiferous aggregates are characterized by a sub- 
continuous. heir occurrence is under such circumstances 1B 
general as to indicate their relation to the single series of dykes, 
