In the determination of the boundary plane for the rocks of a geological province between two great systems, such as the Silurian and Devonian, there are two distinct questions involved:
Conformity to established usage in the application of names and definitions, and
Determination of the correlation of local and definite faunas of the region in question with the particular faunas of the standard sections.
The discussion of the first question must conform to rules accepted by those judging the questions. The general principle of historical priority of usage is accepted by all geologists as a fundamental principle in the application of names and their definitions to faunas, formations, and classification.
TOP OF THE STANDARD SILURIAN SYSTEM, THE “TILESTONES” (=DOWNTON SANDSTONE) OF WALES
According to this general principle, the standard definition of the Silurian system is that applied by . . .