CHAPTER XIII. 



THE JURASSIC PERIOD. 



The eastern part of the continent. — Formations of Jurassic age 

 have not been certainly identified in the eastern half of the continent. 

 Considerable beds which out -crop along the western margin of the 

 Atlantic Coastal Plain have recently been described as Jurassic; 1 

 but this correlation, at least for the upper part of the series involved, 

 cannot be looked upon as probable, much less as established. The 

 lowest of the beds in question (the Patuxent and Arundel formations 

 of Maryland), lying at the base of the Coastal Plain series, are ten- 

 tatively referred to the Jurassic 2 with more reason; but even here 

 nothing has yet been discovered which proves this to be their age. 

 The beds in question are thin (350 feet maximum) and closely asso- 

 ciated with the Lower Cretaceous of the locality where they occur. 

 The basis for their tentative reference to the Jurassic, rather than 

 the Lower Cretaceous, is (1) their unconformity below other Lower 

 Cretaceous beds, and (2) the presence of certain reptilian fossils which 

 are thought (Marsh) to be characteristic of the Jurassic rather than 

 of the Cretaceous. Concerning the first point it is to be noted that 

 there is an unconformity in the Lower Cretaceous above the doubtful 

 Jurassic, so that this argument cannot be said to have great weight. 

 These possible Jurassic beds do not appear to be of marine origin. 



If any of the Coastal Plain beds are to be looked upon as Jurassic, 

 their position and relations emphasize the greatness of the break 

 between this system and the preceding. The Newark series had been 

 uplifted, tilted, faulted and subjected to extensive erosion before the 

 deposition of the doubtful Jurassic beds, which, in their constitution, 



1 Marsh, Am. Jour. Sci.. Vol. II, p. 433, 189G. See also Gilbert, Ward, Hill, and 

 Hollick, Vols. IV and V, 1897. 



2 Clark, Journal of Geology, Vol. V, p. 479. Also Maryland Geol. Surv., Vol. I, 

 p. 190. 



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