The authors thank J. Harold Hudson, Jack L. Kindinger, and Robert B. Halley for aid in collecting samples and for infield discussion of their significance. Randolph P. Steinen, Nealy Bostick, and Judith Parrish kindly reviewed the manuscript and offered comments and suggestions which greatly improved it. Appreciation is extended to Daniel M. Robbin for conducting the majority of the heating experiments, Renato Diaz for making thin sections and printing photographs, George E. Claypool and Ted Daws for LECO carbon analysis, and Charles W. Holmes for constructive discussions. Finally, the authors are indebted to Andrew Strasser for having stimulated our interest in the subject.