What you are seeing is called pounce, which is a find sand like substance used to dry the ink faster. The inks used before the twentieth century did not dry as fast as modern ink, so the pounce material would be used to prevent smearing or smudging when turning the page or folding a letter.

If you look in Monaghan's Learning to Read and Write in Colonial America, she has a brief discussion on pounce powder, it's uses and even gives advertisements in colonial newspapers.

Edit: Since the Wiki page has no sources, check out A Dictionary of English Manuscript Terminology: 1450 to 2000, page 307 where it defines pounce as "a chalky, slightly abrasive powder used by scribes from medieval times until the early nineteenth century...to apply it to the surface of a parchment...in order to remove grease...[and] to absorb excess ink on the parchment or paper page after writing."