Page 21 - visitorGuide

■ Benchmark
Look carefully on the ground about 9 m (30 ft) to your right and you will see
the small bronze benchmark, GSA-1 1973, which was established here by
the U.S. Geological Survey (USGS). It is used by the Survey in its continuing
program of providing horizontal and vertical controls for land surveyors and
for preparation of topographic maps. Inside the front entrance of the build­
ing you can read the 1973 letter from the USGS certifying GSA headquarters
as a permanent benchmark location. The precise location of the benchmark is
40°02′14.53″N,105°14′58.67″W. Its elevation is 5312.874 ft above mean sea level.
The surveys were accomplished by Edwin Eckel, Douglas Hardwick, and Sim Farrow, U.S. Geological Survey, Denver, Colorado.

■ Granitic Specimens

The specimens on the walls next to the main entrance

are two types of granitic rock.The specimen on the left

wall, from the Fletcher-Mason quarry in Mason, New

Hampshire, is gray and white granite cut by a pinkish

band of pegmatite. The specimen on the right wall is a

granitized hornfels from the Cornucopia gold mine in Or­

egon’s Wallowa Mountains. The dark, dense fragments

are hornfels, a metamorphic rock resulting from contact

metamorphism, in this case when shales were “baked”

by the intrusion of a magma body. At one time, this entire

Granitized Hornfels specimen may have been composed of fractured horn­fels.

The fractures likely allowed hot solutions to enter the rock,

chemically altering some of it into granitic rock.

The Mason granite is a gift from Richard H. Jahns, Stanford University, Stanford, California, and a former president of the Society, and

James W. Skehan, Boston College, Chestnut Hill, Massachusetts.The hornfels is a gift from G.E. Goodspeed, University of Washington.

■ Norwegian Blue Pearl Door Pulls
The stone handles on the front door are mostly orthoclase, with minor
amounts of pyroxene, mica, and oxide.The feldspar is iridescent due to micro­
scopic planes of albite which act as a refraction grating.The rock is Larvikite,
an alkalic granitoid classified as either syenite or monzonite, quarried in the
Oslo Graben and cut and polished in Italy.

Gift from Edwin Eckel, U.S. Geological Survey, Denver, Colorado, and former executive secretary of the Society.

Serpentinite Boulder

11OUTDOOR EXHIBITS