POTSDAM PERIOD. 175 



slope of the Green Mountain Range, the Potsdam epoch is supposed to include 

 the lower shales, while the rest of the rocks may he of the Calciferous epoch. 



In Pennsylvania there are 2000 feet of lower slates, overlaid hy 90 feet of 

 sandstone, and this hy 200 to 1000 feet of upper slates (H. D. Rogers). In Vir- 

 ginia there are 1200 feet of lower slates, 300 of sandstone, and 700 of upper slates 

 (W. B. Rogers). In eastern Tennessee Professor Safford has described, as of 

 this age, the " Chilhowee" sandstones and shales, several thousand feet in 

 thickness (consisting of sandy shales, sandstones, and light gray quartzite), 

 resting on the Ocoee conglomerates, sandstones, and micaceous, taleose and 

 chloritic slates. 



c. Eastern border. — On the north side of the Straits of Belle Isle, north of 

 Newfoundland, the Potsdam rocks are gray and reddish sandstone, 231 feet 

 thick, overlaid hy 141 feet of limestone, with some shale, and the latter contains 

 fossils related to those of Georgia, Vt., while in the former there is the Scoli- 

 thus of the New York beds. 



2. CalciferOUS epoch. — a. Interior Continental basin. — In northern New 

 York some of the layers of this Calciferous sandrock are very hard and sili- 

 ceous, and contain geodes of quartz crystals, as at Diamond Rock, Lake George, 

 and Middleville and elsewhere in Herkimer co., etc. The impure limestone 

 layers are adapted for the production of hydraulic lime. The mixture of cal- 

 careous with hard siliceous characteristics is a striking peculiarity of the rock. 

 Owing to the lime present, much of it becomes rough from weathering. Besides 

 quartz and calcite, barytes, celestine, gypsum, and occasionally blende and 

 anthracite, are found in its cavities. 



In Michigan, south of Lake Superior, the Calciferous beds are arenaceous, as 

 in New York, but with some magnesian limestone. Farther west and south 

 the "magnesian limestone" of the epoch is an extensive formation; but it con- 

 tains some intercalated sandstone, and in its lower layers are occasionally geodes 

 of quartz or chert. On the Upper Mississippi, at the Falls of St. Anthony, and 

 also in Iowa, Minnesota, and Wisconsin, the limestone is overlaid by the St. 

 Peter's sandstone (Owen), a friable, incoherent, white rock, affording sand for 

 glass-making. (Whitney.) 



In Missouri there are — (1) a magnesian limestone (No. 1) overlaid by a sand- 

 stone (2), about 350 feet in thickness ; (3) a second magnesian limestone, with 

 some sandstone layers and chert, 500 to 600 feet thick; (4) a sandstone, often 

 cherty and containing quartz crystals in cavities, 70 feet; (5) a third magnesian 

 limestone, cherty and arenaceous, 150 feet; (6) another sandstone; (7) a fourth 

 magnesian limestone. (Swallow.) 



The thickness in Canada is about 150 feet; in New York State, 50 to 300 feet; 

 in Michigan, on the Menomonee (Lake Superior region), 50 to 100 feet; in Wis- 

 consin and Iowa, over 200 feet; in Missouri, between 500 and 1000 feet. 



The magnesian limestones of the Calciferous epoch in the West form bold 

 cliffs along the streams where they have been cut through by running water 

 and other agencies. An analysis of the rock is given on p. 84. It is often 

 oolitic. 



b. Region of tie Appalachians. — The Calciferous beds along the Appalachians 

 have their greatest thickness to the north. At Point Levi, near Quebec, where 

 they have been called the Quebec group, they have been estimated to be 5000 



