CORRELATION OF THE JURASSIC 655 



to be regarded as species evolved under the rather restricted environ- 

 mental conditions obtaining in a small embayment of the Atlantic. 

 Side by side with the European elements in the American fauna lived 

 the species which may be regarded as peculiar to the western hemisphere, 

 although evidently derived from European forms. Thus the Oxfordian 

 fauna of Mexico and Cuba contains Perisphinctes plicatiloides O'Con- 

 nell 19 a species related to, yet specifically distinct from, the widespread 

 P. plicatilis Sowerby and D'Orbigny of Europe and Asia. In the Lusi- 

 tanian and Kimmeridgian there are American species of Ochciocpra.s 

 corresponding to, but not identical with, European species. 20 The Port- 

 landian fauna of Cuba and Mexico contains the typical European genera 

 Simbirskites and Kossmatia, represented, however, by American species 

 (see correlation table). 



The major part of the large Oxfordian fauna of Durango, consisting 

 of twenty species of ammonites described by Burckhardt, comes from 

 the grayish concretions and limestone beds which are intercalated in the 

 lower series of rocks. There is a gradual lithologic change upward in 

 the series, marked by the frequent occurrence of black shales and the 

 appearance of black limestone nodules, which become more numerous 

 as the top of the Oxfordian is reached. There is a complete lithologic 

 transition into the overlying Kimmeridgian (sensu lato) beds, in which 

 the concretions are very numerous, consisting of hard, compact, homo- 

 geneous black limestone and containing Aucellas and many species of 

 Idoceras. The rocks of this age vary from a few meters up to 400 

 meters in thickness in the environs of San Pedro, but to the southeast, 

 at Mazapil, they are only from 15 to 30 meters thick, although of about 

 the same lithic character, with the fossils occurring as impressions in 

 the argillaceous shales and entire in the black limestone nodules. 



The occurrence of these black, bituminous concretions is, therefore, 

 no indication of the age of the rocks in which they occur ; indeed, they are 

 found at successive horizons throughout the entire Jurassic series at 

 Durango, from the higher beds of the upper Oxfordian through the 

 Portlandian — that is,, through a thickness of over a thousand meters — 

 and even on upward into the Cretaceous. In Cuba such concretions are 

 found even lower in the Oxfordian and through all the succeeding 

 divisions of the Jurassic, so that we must conclude that there is no 

 stratigraphic value in the concretions over either a limited or a wide area. 



18 Marjorie O'Connell : The Jurassic ammonite fauna of Cuba. Bull. Am. Mus. Nat. 

 Hist., vol. xlii, 1920, pp. 643-692 (p. 670, pi. xxxvi, figs. 1 and 2). 



20 Marjorie O'Connell : The phylogeny of the ammonite genus Or hctoceras. Bull. Am. 

 Mus. Nat. Hist., vol. 3lvi, 1922, pp. 387-411. 



