- GeoRef, Copyright 2004, American Geological Institute. Reference includes data from Bibliography and Index of Geology Exclusive of North America, Geological Society of America, Boulder, CO, United States
"The revised classification of the Ludlovian rocks is defended. It is maintained that there is no need for separate rock-stratigraphical and bio-stratigraphical classifications. The nine new divisions are, therefore, combined units; their number is justified by the detailed mapping and their names are dictated by the requirements of the rocks and not by an arbitrary code of rules. The old classification, using Aymestry limestone, is considered to be still usable as a crude rock- stratigraphical classification."