Footnote from original book:
Mr. Geo. P. Merrill, curator of the Department of Geology, to whom these stones were submitted, has kindly supplied the following information concerning them:
None of them correspond exactly to the stones used for similar purposes with us. No. 19 is a dark blue-gray, fine-grained argillaceous rock, impregnated with lime, somewhat resembling our ordinary roofing slate, but softer and less finely fissiled.
No.20 is a soft, buff-colored argillaceous rock, more like the German razor hone than anything we use.
No. 21 is a whitish, somewhat decomposed siliceous rock, rather coarse for a whetstone, giving, when breathed upon, a distinct clayey odor, indicating the presence of free argillaceous matter.