CAEBONIFEROUS AGE. 311 



covered with extensive gravel or pebble beds, or deposits of sand ', 

 and these, hardened into gritty rocks, make up the millstone grit and 

 sandstone which underlie the Coal-measures. Similar conglomerates 

 and sandstones were formed afterward in the course of the Coal- 

 measures ; but this rock is prominent for its extent, and for marking 

 the commencement of the Coal era. 



The rocks of the Carboniferous period are accordingly divided into 

 (1) The Millstone grit section ; and (2) the Coal-measure section. 



The Millstone grit extends over parts of some of the southern 

 counties of New York, with a thickness of twenty -five to sixty feet ; 

 and, owing to the regularity of the joints, in Cattaraugus and Alleghany 

 counties, it stands out in huge blocks, walls, and square structures, 

 that have suggested such names as " Rock City " and " Ruined City." 

 It occurs through all the Coal-areas of Pennsylvania, both the eastern 

 and western ; it is from 1,000 to 1,500 feet thick, about the centre of 

 the anthracite region, and diminishes rapidly to the westward. It 

 stretches southwestward through Virginia and Tennessee, to Alabama. 

 Throughout the Appalachians, it is commonly a conglomerate ; but, in 

 the Interior basin, the beds are mainly arenaceous sandstones, and in 

 some parts are absent. 



The Coal-measures include all the kinds of sedimentary rocks : 

 sandstones, laminated or shaly sandstones and shales ; conglomerates, 

 fine and coarse ; buhrstone (a cellular siliceous rock), and limestones. 

 Interstratified with these rocks occurs the coal in layers, and often, 

 also, beds of iron-ore. There is no fixed order of superposition. The 

 following is an example, from Western Pennsylvania, as published by 

 Lesley : the beds are numbered in accordance with their succession, 

 beginning below, — 



Feet. 

 A. Millstone Grit V 



1. Coal No. A, with 4 feet of shale 6 



2. Shale and mud-rock 40 



3. Coal No. B. (Of Mammoth bed of Central Pennsylvania.) . 3-5 



4. Shale, with some sandstone and IRON-ORE 20-40 



5. FOSSILIFEROUS LIMESTONE 10-20 



6. Buhrstone and IRON-ORE ........ 1-10 



7. Shale 25 



8. Coal No. C. The Kittanning Cannel 3J 



9. Shale, — soft, containing two beds of Coal, 1 to 1J feet thick . 75-100 



10. Sandstone 70 



11. Lower Freeport Coal No. D 2-4 



12. Slaty sandstone and shale 50 



13. Limestone 6-8 



14. Upper Freeport Coal, No. E 6 



15. Shales 50 



16. Mahoning Sandstone 75 



17. Coal No. F 1 



