244 PALEOZOIC TIME. 



purer waters. We may hence conclude that the Green Mountain 

 region was a north-and-south island or peninsula, lying between seas 

 of the Connecticut valley and those of New York, and having the St. 

 Lawrence channel on the north. The region of Appalachian sub- 

 sidence, instead of including the Green Mountains, as in the early 

 Lower Silurian era, extended northward, in the direct line of the 

 Alleghanies, over the southern half of central New York, as in parts 

 of the Upper Silurian ; for this is indicated by the position of the 

 sandstone. 



2. FOREIGN UPPER SILURIAN. 



Rocks. — The rocks of the Upper Silurian are widely distributed 

 over the globe, though less universal than those of the Lower Silurian. 

 They occur in Great Britain, Scandinavia, Russia, Germany, Bohemia, 

 and Sardinia, but have not been identified in France or Spain ; also in 

 Asia, Africa, and Australia. They sustain the principle that the 

 earlier formations are in general of continental range. They seem on 

 a geological map to cover but small areas, but only because they are 

 concealed by later formations. 



The Upper Silurian Rocks of Great Britain comprise, commencing with the ear- 

 liest: — 



1. The Upper Llandovery sandstone of South Wales, about 900 feet in thickness, 

 which generally lies unconformably on the Lower Silurian, and its equivalents. The 

 May Hill sandstone of Shropshire, which was first so named, and shown to be Upper 

 Silurian, by Sedgwick. These sandstones terminate in the Tarannon shales, 600 feet 

 where thickest. This group is regarded as the equivalent of the Medina and Clinton 

 groups. 



2. The Wenlock Group, consisting of the gray and black Wenlock shales, 1,400 feet 

 thick, and the Wenlock limestone, 100 to 300 feet thick. They are well exposed between 

 Aymestry and Ludlow, and along Wenlock Edge to Bethel Edge ; also near Dudley, 

 where the Woolhope limestone, a lower part of the series, 50 feet thick, overlies the 

 Llandovery sandstone. The limestone is full of fossils, or rather is made up of them 

 closely compacted; and much of it looks as if it were a deep-water formation. Its 

 American equivalent is the Niagara group. 



3. The Ludlow Group, made up of (1) the Lower Ludlow rock of Shropshire, con- 

 sisting of layers of shale and impure sandstones or mudstones, 900 feet thick; (2) the 

 Aymestry limestone, an impure limestone, 150 feet thick; (3) the Upper Ludlow rock, 

 a shaly or impure sandstone, much like the Lower Ludlow, 900 feet thick. The Tile- 

 stones are series of red and gray sandstones, marlytes and red conglomerates, 1,000 feet 

 thick, regarded as passage-beds to the Devonian. These Ludlow beds and the Tilestones 

 are apparently equivalents of the later half of the American Silurian. There are one 

 or two thin bone-beds between the Tilestones and the Ludlow, consisting of remains of 

 fishes and crustaceans. The limestone of the Upper Silurian fails in North and South 

 Wales, and in some parts even the distinction of Wenlock and Ludlow cannot be made 

 out. 



In Cumberland or Northern England, the Coniston grits, Ireleth slates, and Kendal 

 group correspond to the above groups 1, 2, 3. In Scotland, the lower Sandstone is 

 represented in Southern Ayrshire, and the Wenlock in the Pentland Hills. Upper Si- 

 lurian rocks occur also in Ireland. 



