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MJ 
Esmeralda County, Nevada.—Turner. 263 
close of the Jurassic. In the line of igneous rocks, there is a 
great variety, granite, syenyte, monzonyte, dioryte, diabase, 
gabbro, rhyolyte, dacyte, andesyte and basalt being found in 
abundance, 
The following formations have been identified in Esmeral- 
da county. Some of these will be described in detail under the 
various mining districts in which they occur. 
The Archean Era. 
The oldest rocks of North America are the gneisses and 
schists of the Lake Superior region. These are known to be 
of pre-Cambrian age, and are called typical Archean rocks. 
They are regarded as forming a portion of the primeval crust 
of the globe. In the western United States, Archean rocks 
have been described at many points, but later investigations 
tend to show that some of these areas are of Paleozoic age 
The most certain criterion for the Archean is that of the posi- 
tion below the oldest fossiliferous rocks with an unconformity 
between. 
In the Silver Peak range, underlying Lower Cambrian sed- 
iments, is a complex, the oldest members of which are certain 
gneisses. One of these is a granite-gneiss. Another gneiss 
contains quartz, with both orthoclase and soda-lime feldspar, 
thus approximating to a quartz-monzonyte in composition. It 
may be called a quartz-monzonyte-gneiss. In addition there — 
are granite-augen-schists, and also a_ series of calcareous 
schists, the origin of which is uncertain. These calcareous 
schists are remarkable for the great number of nodules (augen) 
and streaks of, granite which they contain. They usually weath- 
er a dark brown color, strongly resembling an impure limestone 
on the exposed surfaces, yet thin sections of the rock always 
show that it contains much ground up granitic material, as if 
it had been formed by the shearing of a granite, the calcareous 
material possibly having been supplied from other sources by 
infiltration. 
A distinctly later member of the complex is a coarse, white 
granite, sometimes containing muscovite or white mica, more 
often containing little or no mica. At Some points it grades 
over into syenyte by the loss of quartz. This granite is intrud- 
ed into the calcareous schists, above described at innumerable 
