Memoirs of the Geological Survey; Special Reports on the Mineral Resources of Great Britain: Lead and Zinc; The Mining District of North Cardiganshire and West Montgomeryshire. E-book. Formato PDF O. T. Jones   -  Forgotten Books, 2017  - 

It was not until many years later that careful investigation in several distinct localities revealed the true succession of the rocks and the distribution among them Of the fossils (graptolites) which they contain. These investigations first made it possible to trace in detail various rock — groups, and to map the chief axes Of folding and the more important displacements or faults which affect the strata.