[DuncanWaring]

Looking into it a little more rigorously makes it look like that particular phrasing may be a fabrication. However, I've found a book titled the "Illustrated Fryeberg Webster Memorial ", Page 14 contains the passage

...Miracles do not cluster. That which has happened but once in six thousand years cannot be expected to happen often. Such a government, once gone, might leave a void, to be filled, for ages, with revolution and tumult, riot and despotism.

I can't get a usable URL to get directly to it, but if you go to books.google.com and search on "Illustrated Fryeburg Webster Memorial" it'll get you a scanned image version of the book (apparently published in 1882).

[Travis McGee]

I have found it best to go to the scholars who study particular founding fathers and notables for quote attribution. I almost put that bogus Cicero quote into the front of my last book. “Brainyquotes” etc are useless since they verify nothing. Even some alleged old books used as attribution have turne out to be bogus.