THE MA K IXC OF THE GEOLOGICAL TIME-SCALE. 33 



represented around the borders of the Adirondacks, the 

 Chazy, also at the eastern part of the Adirondacks; and the 

 Trenton, expressed typically at Trenton Falls and on the 

 western slopes of the Adirondacks, and extending southwest- 

 ward. 



Silurian System. — The Silurian system, as now re- 

 stricted, is the Upper Silurian of Murchison as defined in 

 1835 to 1838 and later. It includes typically the Mayhill, 

 Wenlock, and Ludlow formations, as defined in the Silurian 

 system of Murchison, of Western England. The fauna is 

 characterized as the " third fauna " of Barrande. It is typi- 

 cally represented in North America by the Niagara, the 

 Salina, and the Lower Helderberg groups of New York State. 



Devonian System. — The Devonian system is a name, also, 

 first proposed and defined by Sedgwick and Murchison in 

 1838. The typical rocks were found in North and South 

 Devonshire, England. The limits of the system, strati- 

 graphically, were not so definitely fixed as in the previous 

 cases, the system having been founded originally on the dis- 

 tinction of the fossils, which by Lonsdale were determined as 

 constituting a group intermediate to the Carboniferous and 

 the Silurian faunas. The fossils from which the original de- 

 termination was made were from the limestones of Plymouth 

 and Torbay, South Devonshire. Later investigations have 

 shown them to be of Mesodevonian age. The Devonian sys- 

 tem was originally intended to include the rock series from 

 the top of the Silurian to the base of the Carboniferous. 

 The lowest member of the system in South Devonshire is the 

 Foreland sandstone, and the highest are the Pilton beds, 

 near Barnstaple, North Devonshire. There are what are 

 known as Lower, Middle, and Upper Devonian faunas, and 

 recent investigations have led certain European geologists* 

 to set the lower limit of the Devonian system low enough to 

 include part of what in North America are called Lower 

 Helderberg group faunas. In America the standard Devo- 

 nian system comprises the Oriskany sandstone, the Cornifer- 



* See Kayser, " Die Fauna der altesten Devon-Ablagerungen des Harzes " 

 (Berlin 187S), and papers on the Hercynian question. 



