A kaolinite mudstone (tonstein) in the British Coal Measures

A bed of kaolinite mudstone, a tonstein, about 2 inches thick was deposited over at least 3,000 sq. miles in the Coal Measures of the Midlands and of the North of England. Based on the field relationships, the petrography, the mineralogy and the geochemistry it is claimed that this band was formed by the in situ alteration of volcanic ash. This tonstein is, therefore, thought o be a bentonite. A review is also given of the literature on tonsteins and postulated origins