Page 57 - visitorGuide

trapped by the algae and a few open pore spaces.The snail shell
is filled with limy mudstone, calcite, and the fossilized shell of
a smaller snail.
Gift from Malcolm P. Weiss, Northern Illinois University, DeKalb.

■ Impact Breccia
Breccias, like this one from the Onaping Formation,
overlie the rich ore bodies of the Sudbury mining
district in Levack, Ontario, Canada.They were once
thought to be volcanic breccias, but they are now

recognized as deposits formed by the catastrophic im-
pact of an asteroid-sized body almost 2 billion years ago.
Fragments of rocks blasted out by the impact fell back into the
crater and were altered and cemented by upwelling magma.
Gift from Bevan M. French, Goddard Space Flight Center, Greenbelt, Maryland.

■ Volcanic Breccia
The rock fragments—or clasts—within this specimen are volcanic in origin.
Their angular edges indicate that they were transported a limited distance
after they were ejected by the volcano.This slab, from the San Juan region of
southwestern Colorado, has been polished to highlight the fragments.
Gift from Edwin Eckel, U.S. Geological Survey, Denver, Colorado, and former executive secretary of the Society.

■ Fossil Nautiloid
These extinct creatures are of Devonian age and lived in a shallow sea (about
350 million years old) where the Atlas Mountains of Morocco are now. They
are distinguished by a straight, rather than curved shell, with a central si­
phuncle and septa curved concave toward the living chamber. This polished
fossil is a relative of today’s chambered nautilus.
Presented to GSA as a centennial birthday gift by the American Association of Petroleum Geologists, through Fred A. Dix.

Fossil Nautiloid

■ Dinosaur Footprint
This is one of three dinosaur
track specimens at GSA from
the Connecticut River Valley
in Massachusetts; the other
two specimens appear on the

wall above the stairwell on the
other side of the third floor. For

more information, see p. 53.
Permanent loan from the Pratt Museum,Amherst Col-
lege,Amherst, Massachusetts, through Gerald P. Brophy.

■ Silicified Wood with Quartz Crystal
Like most petrified wood, this specimen is preserved
primarily by chalcedony, a form of quartz with crystals too small
to see. However, individual quartz crystals are visible on some of the
petrified wood’s surfaces. Visible crystals like these usually form in open
spaces or hollows that existed in the wood when it was buried by sediment.

47THIRD FLOOR EXHIBITS