Prof, James Hall — On the Silurians of the United States. 509 



EXPLANATIOI^ OF PLATE XII. 



Fig. 1. Corynoides gracilis, la. A specimen natural ske. \h. The distal end magnified 



5 diameters. \c. The proximal end mag. 5 dia. 

 Fig. 2. Dendrograptus ramulus. la. A specimen nat. sLse. 25. Part of a branch mag. 



5 dia. 2c. Another specimen nat. size. 



Fig. 3. GraptoUthus attenuatus. 3«. A specimen nat. size. Zb. A portion mag. 5 dia. 



3c. Three hydrothecse mag. 1 dia. 

 Fig. 4. GraptoUthus acutus. ^a. A group nat. size. 46. Part of a specimen mag. 5 dia. 

 Fig. 5. Diplograptus Etheridgii. 5a. A specimen nat. size. 55. and 5c. The distal 



and proximal ends mag. 5 dia. 5d. Another specimen nat. size. 5e. Part of 



a specimen collected by the Gaol. Surv. of Scotland, enlarged. 

 Fig. 6. Diplograptus penna. 6«. A specimen nat. size. 65. and 6c. Portions mag. 5 dia. 

 Fig. 7. Diplograptus pinguis. la. A specimen nat. size. 

 Fig. 8. Diplograptus fimhriatus. 8a. A specimen nat. size. 86. and 8c. The distal 



and proximal ends mag. 5 dia. 8d. Another specimen nat. size. 

 Fig. 9. Diplograptus HincJcsii. 9«. A specimen nat. size. 95. A portion mag. 5 dia. 



9c. An imperfect specimen nat. size. 9d. A young specimen nat. size. 

 Fig. 10. Dicranograptus rectus. 10a. A specimen nat. size. 105. Part of a branch mag. 



6 dia. 10c. The proximal end mag. 5 dia. 



rV. — On the Eelations of the Middle and Upper Silurian 

 (Clinton, Niagara, and Helderberg) Eooks of the United 

 States. 



By Professor James Hall, of Albany, U.S.A. 

 Foreign Member of the Geological Society of London. 



IN the general classification of the Palceozoic Eocks of North 

 America, while we have carefully compared and attempted to 

 correlate them with the greater divisions of the Silurian, Devonian, 

 and Carboniferous systems of Great Britain and the continent of 

 Europe, we have still retained the local names of formations, which 

 to a great extent were given in the Geological Survey of New 

 York. These names, like English local names, indicate a locality 

 or region where the strata are well shown, and the characteristic 

 features of the formation better developed than in other places. 

 Several of these local formations have thus been grouped together as 

 representing certain stages in the systems of formations as recog- 

 nized above. 



In this grouping together we have the Medina Sandstone, the 

 Clinton Group, and the Niagara Group as representatives of the 

 Middle Silurian, while we arrange the Water-lime and Lower Hel- 

 derberg Groups in the Upper Silurian strictly. These two divisions 

 are clearly separated from each other by the Onondaga Salt Group 

 as shown in the subjoined diagram (1) thus : 



TT S'l 'an I Lower Helderberg Water-lime. 



" ( Onondaga Salt Group or Salina formation. 



( Niagara Group. 

 Middle Silurian < Clinton Group. 



( Medina Sandstone. 



/ Hudson Eiver Group. 



-r a-i • I Trenton 



Lower Silurian .. 



1 Trenton J 



j Black River > Limestones. 



\ Birdseye ) 



Below these come the well-marked Lower Silurians, the equiva- 

 lents of the Caradoc and Llandeilo formations. 



