so extremely old as seeming to belong to an earlier period; of or relating to the period before the biblical flood
This is from Webster. I know "so as to do...", but what is this "so~as doing~"? is this correct?
This is a variant of the so ADJ that [clause] comparative construction:
Here that is replaced with as because [clause] is expressed with a participle, a non-finite form, rather than a finite verb. (As FumbleFingers points out, the same substitution may be made when the verb is an infinitive.) It may be paraphrased
This is a very rare use; it occurs here only because when M-W defines an adjective or other modifier it takes great pains to avoid mentioning the entity which is modified—in this case, it. That is to my mind a very silly constraint, and I think you can safely leave this awkward construction out of your own repertory.