So I started trying to find an older source for this Cotton Mather quote. And I can only find one reference to the quote before 1900, and it is clearly not from Cotton Mather. It appears in various editions of Hubert Howe Bancroft's The Native Races, such as this footnote starting on page 74 of the 1886 edition:
When the taste of American antiquaries inclined towards Phoenician relics, the Dighton inscription conformed to their opinions; and with changing tastes it has proved equally compliant. In 1783 the Rev. Ezra Stiles, D.D., President of Yale College, when preaching l>efore the Governor and State of Connecticut, appealed to the Dighton Rock, graven, as he believed, in the old Punic or Phoenician character and language: in proof that the Indians were of the accursed seed of Canaan, and were to be displaced and rooted out by the Kuroiiean descendants of Japliet!.... So early as 1080 Dr. Danforth executed what he characterized as "a faithful and accurate representation of the inscription" on Dighton Hock. In 1712 the celebrated Dr. Cotton Mather procured drawings of the same, and transmitted them to the Secretary of the Koyal Society of London, with a description, printed in the Philosophical Transactions for 1741, referring to it as "an inscription in which are seven or eight lines, about seven or eight feet long, and about a foot wide, each of them engraven with unaccountable characters, not like any known character."
As you can see, the quote is from Ezra Stiles, Yale's president--not Cotton Mather. Is anyone aware of an earlier use of this quote, and specifically by Mather? The paper is about the Pequot War, a subject on which I know quite a bit, and it does not fit at all.