114 GEOLOGY. 



local conditions which allowed of the differentiation of sediments 

 to an unusual degree. This differentiation may have been effected in 

 large part by land drainage. If marshes, lagoons, and small isolated 

 bodies of water were the sites of deposition, and if the contributing 

 streams were of varying velocities, and therefore bearing loads of vari- 

 ous grades of coarseness, some of the peculiarities of structure would 

 find their explanation. Slight oscillations of level, or slight shif tings 

 of the debouchures of the streams may have caused the deposits of 

 separate streams to become continuous. Similar results might have 

 been brought about if the conditions were estuarine. If this was the 

 case, there must have been a barrier to the east, shutting out the sea, 

 and of such a barrier there is some evidence. 1 



In addition to the clastic sediment, there is a little lignite, and 

 some iron ore, and though both are widely distributed, neither is of 

 much commercial value. Both formations are natural results of the 

 conditions assigned. Amber has been found in the series at several 

 points, though in small quantities only. 2 



The Tuscaloosa series is like the Potomac in general constitution, 

 though gravel is, on the whole, less important. Clay predominates 

 in the lower portion, and sand in the upper. The bright colors and 

 the irregular stratigraphy characteristic of the Potomac are also char- 

 acteristic of the Tuscaloosa series. 



The Potomac, as already implied, is a series of formations, rather 

 than a single formation. Even if the lowermost part of the series 

 heretofore called by this name proves to be Jurassic, the portion above 

 is not a unit. In Maryland 3 two distinct formations (the Patapsco 

 and the Raritan) have been recognized within it, the one unconform- 

 able on the other. A similar subdivision has not been established for 

 the series farther south. 



Stratigraphic relations. — Along the Atlantic Coast the Potomac 

 series rests unconformably on the Triassic (New Jersey) and pre-Cam- 

 brian (Pennsylvania and south) formations. Its general strati- 

 graphic relations are shown in Fig. 380. The Tuscaloosa series rests 

 on crystalline schists (pre-Cambrian) at the east, but farther west on 



1 Clark and Bibbins, Bull. Geol. Soc. of Am., Vol. XIII, pp. 209-12. 

 2 Hollick, Am. Nat., Vol. XXXIX, 1905. 



3 Clark, Jour, of Geol., Vol. V, 1897, pp. 479-506; also Maryland Geol. Surv., 

 Vol. I, pp. 191-2. 



