SUBDIVISIONS OF CAEBONIFEBOUS. 109 



4. The Alternating Series. — A series of alternating beds of pink and light 

 colored sandstones and limestones somewhat argillaceous ; the lime- 

 stones apparently somewhat magnesian and poor in fossils 150-200 



These pass upward into a variegated sandstone, irregularly stained red- 

 dish, yellowish, and white; somewhat argillaceous and calcareous and 

 exhibiting little stratification 5 weathering in peculiar twisted, contorted 

 (curly) bands 100-150 



Wherever the Potsdam sandstone and lower Carboniferous series 

 are together visible they are absolutely conformable, and the distinction 

 between the rocks of the two systems is often so perfectly marked that the 

 blade of a penknife may be inserted at their line of separation. Passing 

 upward from the coarse brown sandstones of the Potsdam, with Lingulepis, 

 Obolella, etc., one first meets the thinly bedded pinkish limestones of the 

 base of the Carboniferous with fossil forms characteristic of that formation, 

 so that both lithologically and paleontologically the separation between the 

 Potsdam and the Carboniferous is always distinct and easily traceable. 

 This lowest member of the Carboniferous weathers easily and is not gener- 

 ally well exposed, though the separation between it and the heavy limestones 

 above is very distinct where observable. It has a pinkish or grayish color, 

 is never a pure limestone, and commonly has an argillaceous appearance. 

 Its strata rarely exceed eight or ten inches in thickness, and it is often so 

 thinly bedded as to resemble a calcareous shale. Though it is commonly 

 very fossiliferous, specimens sufficiently well preserved for specific identifi- 

 cation were not obtained. The genera Zaphrentls, Spirifera, and Productus 

 were recognized. 



The second member, the massive gray limestone, and the third, the 

 silicious limestone, are the most persistent features of the formation in the 

 Hills. They are well seen in nearly all parts of the country examined ; in 

 the western plateau, on the southern border, and on the eastern slope. 

 Along the eastern edge of the plateau, however, there are considerable 

 areas from which the silicious limestone and part of the gray have disap- 

 peared by denudation; and for this reason one who enters the Hills by 

 crossing the plateau from the west obtains but a very inadequate idea of 

 the true magnitude and thickness of the Carboniferous formation. Com- 

 plete and continuous sections of the Carboniferous are found only in the 



