334 H. S. WILLIAMS — SILURIAN-DEVONIAN BOUNDARY 



1. Conformity to established usage in the application of names and 

 definitions, and 



2. 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 Silu- 

 rian system is that applied bj' Murchison to the system so named by him 

 in the classic monograph called u The Silurian System," published in 

 1839. By this standard the upper limit of the Silurian system is fixed 

 for all future workers. This upper limit was originally placed at the 

 base of the " Tilestones " (Downton sandstone), the highest Silurian for- 

 mation being the upper Ludlow rock, near Downton castle.* The " Tile- 

 stones," lying immediately above it conformably, were then included in 

 the Old Red sandstone.f 



In later elaborations of the system the " Tilestones " were transferred 

 to the Silurian because their fossil contents agreed in the main with the 

 fauna of the Ludlow rock below ; and Murchison made the explanation 

 that originally it had been associated with the " Old Red " because it 

 was often of reddish color and decomposed to a reddish soil. J To this 

 have been added the Ledbury shales of Salter, some 300 feet of red, 

 gray, and purple shales and sandstones, containing Pterygotus and 

 Cephalaspis,§ which by geologists now are recognized as included with 

 the Ludlow rock in the upper part of the Silurian system of Wales. || 

 Not only Merostomes (Eurypterus and Pteygotus) and fishes (Onchus, 

 Cyathaspis, and Pteraspis), but also land plants are reported from these 

 upper beds of the Silurian system. The marine invertebrate faunas 

 cease at this point in the Welsh section, and are followed by Old Red 

 sandstone. 



BASE OF THE TYPICAL DEVONIAN SYSTEM NOT KNOWN 



The Devonian system was established on a different basis. The fos- 

 sils collected at Plymouth, Torquay, etcetera, in South Devonshire, were 



*See Silurian System, p. 197. 



t Loc. eit., p. 181. 



tSiluria, ed. 18. r i4, p. 138. 



§ See Quarterly Journal, vol. xvi, p. lit:'.; vol. xvii, p. 152. 



|| Kayser : Text-book of Comparative Geology, trans, by Lake, 1&93, p. 65. 



