THE MAGISTRAL DISTRICT, STATE OF JALISCO. 395 
eral geological outlines to those deposits occuring entire- 
ly in intrusive rocks. Such deposits occur in the Sierra Ne- 
vada Range of California, in Idaho and in some parts of New 
Mexico, while those of Butte, Montana, are typical of the 
class. The predominant country rock of the Magistral Dis- 
trict, is a large body of diorite, showing wide variations as 
to physical structure and character. This variation extends 
from a coarsely grained diorite of granitic structure on 
one end, to an even grained, micro-diorite, with frequent 
changes to porphyrites on the other. 
This very varied country rock is cut in several places 
by thick dykes of quartz porphyry. The veins occur on or 
near the contact of these dykes with the diorite and almost 
parallel them in strike. Although the veins show in several 
places, especially near the surface, indications of their 
being fractures filled with quartz and rock fragments, the 
deposits are more commonly closely connected with ancient 
fault planes, along which quartz with pyrite and varying 
amounts of chalcopyrite seem to have impregnated the 
brecciated friction zone of the fault, with replacement occu- 
ring on either one or both sides of the fault. Due to intense 
silification, the ores show all grades, from a purely quartzose 
mass to hard fragments, in which the original rock may 
be identified. It is not uncommon to find pyrite occuring in 
small masses or grains in the comparatively soft, altered, 
greenish rock, of frequent occurence in a number of cop- 
per districts. In so far as is at present known, the dykes 
have exerted no apparent influence on the veins and the min- 
eralization has not been found to extend through the dyke 
rock, 
The outerops of the Magistral Veins, well marked and 
in some cases prominent, consist of silicified diorite, oc- 
casional quartz stringers and specular hematite. No true 
