METASOMATISM IN CONNECTION WITH VEIN FORMATION. 
187 
Serpentine develops in places from ferromagnesian silicates under the influence 
of incipient vein action, but is probably soon altered to carbonates. 
Adularia is a pure potassium feldspar which occurs in some mineral veins. ° 
The Cripple Creek deposits contain a considerable amount of this mineral, both as 
fissure-filling and more prominently as a metasomatic product . b It nearly always 
shows crystallographic outlines, and as the forms are combinations of the prism 
and a dome, the outline of the crystals usually shows a rhombic, somewhat elon¬ 
gated form. Larger grains often have peculiar optical irregularities, somewhat like 
that of the quartz in the filled veins of Cripple Creek (p. 179), the mass being 
divided into several irregularly defined sections with slightly differing optical 
orientation. 
Adularia forms in all the various rocks, most abundantly, however, in the 
metasomatically altered granite, the so-called granite ore. In little veinlets its 
crystals coat the walls as they do in cavities of dissolution, where the central part 
is apt to consist of dolomite or fluorite with pyrite. In the mass of the rock it 
occurs as rhombic sections or is attached to old orthoclase crystals, as marginal 
growths with parallel orientation or healing fissures in the same. Sometimes the 
distinction from the primary orthoclase may be difficult, but the more recent min¬ 
eral is usually very clear and free from inclusions. In the granite much of the 
older feldspar is microcline, but the secondary mineral never belongs to this species. 
It is somewhat surprising to see sericite and adularia develop in the same rock, 
side by side (PI. XVIII, B ); in some instances, indeed, fibers of sericite are devel¬ 
oped in a mass of adularia. However, the principal part of the adularia is confined 
to the granite, while in the volcanic rocks sericite is more abundant. To a certain 
extent the formation of adularia is due to the prevalence of the orthoclase in the 
original rock, for, as pointed out by Van Hise, c the mineral species most abundantly 
present have the advantage over other kinds of minerals, which are absent or spar¬ 
ingly present, in determining the character and the orientation of new additions. 
Thus, quartz will develop most abundantly in siliceous rocks and orthoclase in 
those rich in that kind of feldspar. 
Quartz is formed during the alteration of sericite from orthoclase and during 
the decomposition of ferromagnesian silicates, but it appears to be carried away in 
soluble form. At any rate, quartz is not of general occurrence as a metasomatic 
product, though it is often deposited on fissures. Quartz crystals are seldom 
deposited in the small cavities of dissolution so common in the volcanic rocks. 
Adularia is far more common. Some occurrences of silicified rocks will be described 
below, but they occupy a rather exceptional position. 
Dolomite and calcite are common constituents of the altered rocks of all kinds. 
Dolomite replaces the cementing mass in breccia, the ferromagnesian silicates in 
latite-phonolites, and develops almost as easiH in the orthoclase. The dolomite 
always tends toward a crystalline development, the form being a more or less 
roughly outlined normal rhombohedron. Siderite develops from magnetite in small 
quantities. 
a Lindgren, W., Orthoclase as a gangue mineral in a fissure vein: Am. Jour. Sci., 4th ser., vol. 5. pp. 418-420. 
b Lindgren, W., Metasomatic processes in fissure veins: Trans. Am. Inst. Min. Eng., vol. 30, 1901, p. 612. 
c A treatise on metamorphism: Mon. U. S. Geol. Survey, vol. 47, 1904. p. 122. 
