168 PALAEOZOIC TIME — SILURIAN AGE. 



in the State of New York, where the strata are well displayed and 

 have been carefully studied. 



Some standard for our divisions of time must be adopted; and, whatever that 

 standard, it is afterwards easy to compare with it, and bring into parallelism, 

 the successive strata or events of other regions. The State of New York lies on 

 the northeastern border of the great interior, — a vast region stretching south- 

 ward and westward from the Appalachians, to the Rocky Mountains, and 

 beyond the head-waters of the Mississippi to the Arctic Ocean, over which 

 there were many common changes ; and, owing apparently to this situation 

 on the north against the Azoic, and near the head of the Appalachian range, 

 there are indicated a greater number of subordinate subdivisions in the rocks, 

 or of epochs in time, than ai'e recognized to the west. It is, therefore, a more 

 detailed indicator of the great series of changes and epochs in the Palasozoic 

 era, and hence it is especially well fitted to become the basis of a scale or 

 standard for the subdivision of time. This will be apparent in the course of the 

 following pages. 



The order of succession in the Silurian periods and rocks is 

 shown in the section on page 131 (fig. 134). The numbers affixed 

 to the subdivisions of the section are used for the same formations 

 throughout the work. 



Subdivisions of the Silurian. 



II. UPPER SILURIAN. 



3. Lower Helderberg Period (7). 



Lower Helderberg limestones, including in New York (1) 

 the Water-lime group ; (2) the Lower Pentamerus limestone ; 

 (3) the Delthyris shaly limestone; (4) the Upper Pentamerus 

 limestone. 



In Great Britain, the Ludlow beds, including the Lower Ludlow limestone, 

 the Aymestry limestone, Upper Ludlow limestone. In Norway, Upper Malmo 

 limestones and schists. 



2. Salina Period. (6.) 



Onondaga Salt group. 

 1. Niagara Period (5). 



4. Niagara Epoch (5 d) : Niagara shale and limestone. 



3. Clinton Epoch (5 c) : Clinton group. 



2. Medina Epoch (5 b) : Medina sandstone. 



1. Oneida Epoch (5a): Oneida conglomerate. 



In Great Britain the Wenlock shale and limestone are referred to the Niagara 

 (5 d) ; the Upper Llandovery, to the Clinton (5 c) ; and the Upper Caradoc sand- 

 stone, Coniston grits, Lower Llandovery, to the Medina and Oneida epochs (5 6 

 and 5 a). Murchison divides the Upper from the Lower Silurian of Britain at 



