It is indeed Mark's peculiar reproduction of Is.xxix.13, in opposition to the original and the LXX.

A further proof that the Roman Christian has here our Synoptic text in his mind, may be taken from c.

xiii, where he quotes Jer.ix.24 with equal divergence from the LXX, after the precedent of the Apostle (1 Cor.i.31, 2 Cor.x.

17) whose letters he expressly refers to (c.

xlvii) [Endnote 69:1].
It is difficult here to avoid the conclusion that Clement is quoting the Old Testament through the medium of our Gospels.