The text of this manuscript was converted to postscript from a WordPerfect file but should be readable. The figures are included in separate files. Figures 1 and 2 are have not yet been scanned in; if you really need them, or if you have any other questions or comments, please contact me.

Nick Street
nick-street@uiowa.edu

manuscript

figure 3

figure 4

figure 5

figure 6