For mercies countless as the sands. J. Newton. [Praise.] Appeared in the Olney Hymns, 1779, Book i., No. 50, in 5 stanzas of 4 lines, and based upon Psalms cxvi. 12,13. Its use both in the older, and in modern collections both in Great Britain and America, is extensive. Original text in Hymnal Companion, No. 501.
The authorship of this hymn is sometimes attributed to W. Cowper, the poet, but in error. It is not given in any of the collected works of Cowper, and his signature in the Olney Hymns "C" is not added to this hymn, in any edition with which we are acquainted, and certainly not in the first of 1779. We attribute it to J. Newton on the ground that all unsigned hymns in the Olney are claimed by him.
--John Julian, Dictionary of Hymnology (1907)