Okay, so I just noticed something. There's the 3-lined x things and there are also 4-lined x things. According to a list of alchemical symbols I found, the 4-lined x things represent Salammoniac. According to Wikipedia, Sal Ammoniac (the only thing that came up when I googled it) is:
"It typically forms as encrustations formed by sublimation around volcanic vents. It is found around volcanic fumaroles, guano deposits and burning coal seams. Associated minerals include sodium alum, native sulfur and other fumarole minerals. Notable occurrences include Tajikistan; Mt. Vesuvius, Italy; and Parícutin, Michoacan, Mexico. "

Perhaps this is the stuff, or at least some of the stuff that was surrounding the vents?