204 The American Geologist. April, 1894 
sheets in the Triassic sandstone of New Jersey pol only impregnate, but. 
apparently, actually replace the sandstone. 
(g) Metasomatic deposits in igneous rocks. The first examples noted 
occur in volcanic rocks, and the later ones in plutonic rocks. In the 
cupriferous amygdaloid of Keweenaw Point the copper is in pari a true 
replacement of the diabase. The auriferous quartz of the Little Annie 
mine. Rio Grande county. Colorado, is probably a silicified rhyolite. At 
Silver ( 'lid'. Colorado, rhyolite is impregnated with cerargyrite, and in 
the Calico district of California liparite and tuff are similarly impreg- 
nated. The copper deposits of Llano county, Texas, are partly fissure 
veins, but chiefly true Impregnations in granite. The copper ores of the 
Morenci district, Arizona, are in pari impregnations of the porphyry in 
which they occur. Many of the gold and silver deposits of the Rockj 
mountains, etc.. are metasomatic impregnations of dikes and plutonic 
masses. The tin deposits of Irish Creek. Ya.. are typical impregnations 
of granite among leaders (veinlets) of quartz. 
(h) Metasomatic deposits in metamorphic rocks (gneisses and schists). 
These deposits, when conforming with the bedding or foliation, consti- 
tute the typical fahlbands; and are then not easily distinguished from 
original sedimentary deposits on the one hand, and interbedded veins on 
the other. Besides the classic example, the argentiferous fahlbands at 
Kongsberg, Norway, which are of economic interest only at their inter- 
sections with a series of veins, we may notice one important American 
locality. The greal pyritiferous bed (superficially oxidized) of the 
Homestake mine and other similar beds in the Black Hills, are either 
original or secondary (true) fahlbands. If the pyrite is a part of the 
original sedimentary deposit, the subsequent changes — crystallization 
and segregation — have probably been sufficiently extensive to justify 
the present classification. 
3. Substitution deposits, or complete metasomatic replace- 
ments. — Through the recent labors of Dr. S. K. Emmons and 
others, the substitution deposits have been proved to be of 
great importance; and many deposits formerly classed as 
veins are now known to be complete metasomatic replacements. 
Metasomatic impregnations may be regarded as marking, in 
general, the halfway point in the development of substitution 
deposits; although in calcareous rocks the impregnation stage 
can not be regarded as essential, the contact between the ore 
body and the limestone being often sharply defined. Like im- 
pregnations, the substitution deposits may or may not occur 
in the walls of veins or along igneous contacts. Substitution 
deposits may be divided in the same way as impregnations: 
but since they present comparatively little commingling of 
rock and ore, it is sufficient for our purpose to recognize two 
subtypes, as follows : 
