Cambrian Brachiopoda

The base of the Paleozoic and of the Cambrian has been placed at the lowest known limit of the Olenellus fauna, all clastic rocks beneath being referred to the Algonkian,* a name given to the period embracing the time of the deposition of those clastic rocks which are older than the Cambrian.† The base of the Algonkian is the lowest of the recognizable clastic rocks. This base is easily recognized beneath the Belt series of rocks in Montana, in the Grand Canyon region of Arizona, in portions of the Lake Superior region, and in eastern Newfoundland, as there are great unconformities between the Algonkian and the Archean, above which the clastic rocks are clearly defined. In other regions, however, the delimitation of the Algonkian and the Archean is very difficult. The basal plain of the Algonkian is obscured by volcanic rocks which, with the clastics, have been so altered . . .