For those who are curious, here is the Mass of the Feast of Saint Eugenius at Saint-Eugène-Sainte-Cécile (you can hear the introit at 18:55): https://www.youtube.com/watch?v=P2I0YAhc2Eg&t=2252s You will notice the Mass is sung in Parisian plainsong, which is quite different from Gregorian chant.
This gorgeous alternation between organ and voice at 37:27 reminds me of this Parisian plainchant album from the esteemed Ensemble Organum - probably results in one of the longest Glorias I've ever encountered (20min!) https://youtu.be/XR6UNfm26q4
Also has a piercingly lovely Christmas prose Votis Pater annuit with a similar 6/8 feel. https://youtu.be/ahcfhsP4ZEg
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