Dr. George G, Shumark, of, Arkansas, re-cently delivered a speech upon railroad matters at Fort Smith, Ark., during which he made known the very important fact that the largest gypsum field in the world lies about three hundred miles west of Fort Smith, Ark., in the plains explored by Capt. Marcy last year, extending over an area of three hundred miles north and south, east and wst. The strata in some places is twenty feet thick, of the purest kind, white, and in some instances transparent. He says there is a sufficient quantity of it to supply the whole world, and would employ a railroad in its transportations one hundred years. Gypsum when burnt, becomes what is known by th name of plaster of Paris—a very valuable article.