Three strata of toadstone had been found, and the limestone under the third was as far as miners had reached. The map project describes the "shallow tropical sea surrounded by coral reefs" in which the limestones were laid down and how "subterranean volcanoes erupted spasmodically through the sea floor, covering the limestones with ash and lava" which formed the toadstone.
is Galena (lead sulfide) formed as pockets or veins in carbonate rock into which the mineral bearing fluid rock has intruded.
All plants were responsible for the transformation, but Archaeopteris was important because it made up 90 percent of the forests during the last 15 million years when these changes accelerated" ...
"It was the first plant to produce an extensive root system, so had a profound impact on soil chemistry".
The end-Frasnian extinction (the largest) about 375 million years ago.