422 The- Upper Devoxiax Deposits of Marylaxd 



formation, often rendering it very difficult to fix upon a precise boundary 

 between tlie Jennings and Catskill. Tn the area west of Sideling Hill 

 a thick red bed which resembles Catskill sediments in all respects, makes 

 its appearance about 4000 feet above the base of the Jennings, and is 

 in turn overlain by deposits of Jennings type carrying Chemung fossils. 

 Persistent Catskill sediments reappear 4700 to 4800 feet above the base 

 of the Jennings. Eastward the Catskill thickens and the red strata ap- 

 pear lower and lower in the section. The relations believed to exist are 

 illustrated in the following diagram : 



West Sideling Hill East 



~ Sediments of Jennings __— — ' Sediments of Catskill 



type ^ ^-^-—' t_yP« 



In view of the foregoing facts the question arises. What is the upper 

 limit of the Jennings formation? It is well known tliat the Chemung 

 and Catskill are not successive formations but are, to a considerable 

 extent, of the same age, representing two phases of sediments deposited 

 at the same time but under different conditions, the Catskill phase 

 accumulating more largely in the east, while the Chemung phase has 

 its larger development farther west. It is hence impossible to discrimi- 

 nate them upon the basis of age. It is manifest that Catskill and Jennings 

 conditions alternated in the same region, due probably to oscillations of 

 level, so that sediments of Jennings and Catskill type intertongue re- 

 peatedly, the Catskill beds thickening eastward and the Chemung beds 

 thickening westward. 



The line of division between the Catskill and Chemung is hence arbi- 

 trary to a large degree and expresses the opinion of the observer. The 

 method used in discriminating these formations in Marjdand has been 

 to draw the top of the Chemung at the upper limit of abundant marine 

 fossils, which is also in general at the base of the more persistent red 

 beds. This is the most trustworthy and hence the most satisfactory 

 method to employ in this area, in the opinion of the writer, since the 

 highest marine fauna is of undoubted Chemung type. 



