It's a geological "sequence"; that is, it is the order in which rocks were formed. To call the pre-Isuan a sequence is pushing it a bit far probably, since they don't have much evidence for its existence, never mind a sequence. But I guess there logically must have been one regardless of whether or not they can find any evidence!

Be interested to hear what your tame professor says, WW.

Getting on for 25 years since I studied geology, though. I do try to keep up, but they find new stuff alla time ...