SEDIMENTARY FORMATIONS. 25 
CHARACTER. 
The Homestake limestone is a dark bluish-gray limestone of a 
dense texture, with uniform characteristics throughout its entire 
extent, except near the laccolith contact. Under the microscope 
the limestone appears to be made up of exceedingly minute grains 
of calcite with scattered grains of pyrite, magnetite, and chert. 
The bedding of the Homestake limestone is very indefinite and is 
easily confused with secondary fracturing. Where it is well defined 
the limestone is generally thin bedded. 
CONTACT METAMORPHISM OF LIMESTONE B\ ANDESITE LACCOLITHS. 
Phases of alteration.— -The limestone adjacent to the andesite has 
been locally replaced by iron ore and has been generally vitrified, sili- 
cated, and kaolinized in a band usually not more than 60 feet wide 
along the erosion surface, although locally it may be a few hundred 
yards wide where the erosion surface is nearly parallel to the lime- 
stone-andesite contact. Locally either or both contact phases are 
absent. 
The altered limestone is a grayish, yellow, or greenish, fine-grained, 
argillaceous-looking rock. Near the contact it is soft, and farther 
away it is hard and fractured into small irregular blocks. The prin- 
cipal minerals are albite, kaolin, actinolite, diopside, quartz, ortho- 
clase, serpentine, phlogopite, andradite, iron ores, osteolite, andalusite, 
wollastonite, calcite, etc., varying greatly in proportion in different 
places, but usually occurring in quantity in the order named. They 
are found in veins, in breccias, and disseminated through the rock. 
In addition there are local residues of a glassy base. The albite is 
probably not as abundant as here indicated, but very likely includes 
other sodium silicates which have not been detected. The glass can 
be distinguished only with difficulty from opal and other isotropic 
minerals. Its index of refraction was determined as 1.56 by means of 
the Becke method used in conjunction with liquids of known index of 
refraction. This distinguishes it from opal (1.45) and other isotropic 
minerals which might be found in contact-metamorphic limestones. 
Another phase is coarsely crystallized limonite-stained marble, in 
some places found in a narrow belt between the andesite and the nor- 
mal silicated contact phase and elsewhere outside of the normal phase 
or associated with the ore. It is thought possible that this limonitic 
marble is a later vein material, filling openings along the contact left 
by the cooling and crystallization of the intrusive and intruded masses. 
Analyses of various phases of the Homestake limestone are given 
on the following pages. 
