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.
By carboniferous [carbon bearing] soils, Richard Kirwan in 1799 meant the "various sorts of earth or stone among or under which coal is usually found".
It is suggested that analysis of sedimentary pyrite from the Belingwe sedimentary basin in Zimbabwe may provide evidence for bacterial reduction at this date.
2,500,000,000 to 542,000,000 years ago The Proterozoic Eon: the most recent part of the Precambrian Supereon.
Galena can contain minor amounts of antimony, arsenic, bismuth, cadmium, copper, silver, and zinc See and stalagmites.
Encountering impermeable strata (such as toadstone) it forms underground lakes in the limestone which overflow to the surface as natural springs (a resurgence).
starting about 2.500,000,000 years ago, sedimentary rocks affected by higher amounts of oxygen in the atmosphere, appear red, as though rusted. Fossils include earliest fish, but of types very different from modern ones. signs of the "tentative presence" of plants on land.
Mesoproterozoic from 1,600,000,000 to 1,000,000,000 years ago. Particular importance attaches to occurred around 1,300,000,000 years ago during the late Mesoproterzoic era, after the earth's transition to a more highly oxygenated atmosphere with an ozone screen. Deposits that made Westmorland Green slate formed around 450 million years ago. About 50 million years later, the material was altered by heat and pressure (metamorphosed) to slate, during mountain-building. After many more millions of years, they resembled patches of moss a few centimeters high.
The end-Frasnian extinction (the largest) about 375 million years ago.
The about 365 million years ago during the Famennian.
writes of fine-grained greenish sandstones deposited in freshwater in which land plant fossils are well preserved.
"Among the most attractive of these" tree-fern once called Cyelopteris (Round-leaved Fern), re named Palasopteris Hibernicus (Primitive Irish Fern). It was the "monarch of the primeval forests" whose "graceful fronds bent over the clear waters of a lake".
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" ...