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March-April 2012

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An Arizona Clam Dig, Carnelian-Replaced Clams From the Petrified Forest Area, Apache County, Arizona

Extensive lowlands dominated the Arizona landscape during the Triassic Period. Forests with giant trees and streams provided a variety of habitats for both plants and animals. Early dinosaurs left their bones and tracks in the Triassic sediments. Remnants of this environment include giant conifer trees and lesser-known invertebrates that can now be found as fossils weathering out of the sedimentary rocks in and around the Petrified Forest National Park.

A new and unique find of carnelian-replaced freshwater clams was recently made east of the Petrified Forest National Park in Arizona. Approximately three hundred specimens were collected from a small, flat-lying outcrop about 9 meters across that is now exhausted. The clam shells vary in preservation from a recrystallized pale gray calcium carbonate with only a thin shell of silicified material on the outer shell, to shells that are completely replaced and filled with a brilliant, finely banded, red carnelian agate with shades of orange and yellow and occasionally blue and violet. Many of the agates have a hollow cavity in the center lined with crystals of calcite, barite, and quartz with a smoky or amethyst hue.

Frank Bain, a professional exploration geologist and paleontologist for more than thirty years, has worked for numerous mining companies worldwide. He is an avid mineral and fossil collector and has led and participated in numerous paleontological digs sponsored by the National Geographic Society and the Los Angeles County Museum of Natural History.

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