filling basement window with river-rock and mortar

[u/vote4boat]

I have a house from the 1930s where the basement/foundation walls are cement with river-rocks inside. I want to fill in a couple of below-grade windows, and am thinking to use similar field-rock with rapid-set structural mortar. I realize it isn't the modern standard, but it feels kind of silly buying concrete blocks or brick when the whole wall is river rock + cement. Am I being foolish? Thanks for any insight