The Ammonite Zones of the Lias

Extract

The increasing diversity of interpretation of the Jurassic Record is proving very troublesome to teachers and students alike. “To-day, hardly a subdivision of the Jurassic is free of controversy as to its exact scope and nomenclature,” wrote S. W. Muller in a recent laudable attempt to reconcile the differing opinions. More is involved, however, than a mere reconciliation of conflicting views. Our knowledge of the Jurassic Record is as yet far less complete than many geologists realize. For while some of the subdivisions of Jurassic time are in general use by all stratigraphers and have stood the test of time, others are in dispute because they are still based on too local and often defective material, or on too incomplete information; a number remain as much surmises as when they were originally introduced. Any scheme put forward at the present day cannot thus be regarded as the final word in the subdivision of the Jurassic, but merely as a contribution towards an ultimate, ideal, “Standard.”