128 L. F. Ward — Evidence of the Fossil Plants 



This presentation brings out in a clear light the value of 

 these species in determining the age of the Potomac formation. 

 Just half of them are found in the -Wealden, and all but three 

 occur below the Cenomanian. 



Two problems of considerable importance remain to be con- 

 sidered, viz : First, how much weight to give to the large num- 

 ber of Jurassic types as indicating a Jurassic age for the form- 

 ation, and secondly, how much to give to the Dicotyledons as 

 pointing to a middle Cretaceous age. 



As regards the first, we find that eight ferns, four cycads, 

 and two conifers have representatives in the Rhetic, which some 

 regard as uppermost Trias ; four ferns and two cycads in the 

 Trias, or lower Jura ; 24 ferns, two cycads and five conifers in 

 the Oolite, and two ferns and eight conifers in the upper Jura 

 (Corallian and Kimmeridgian). These species, of course, over- 

 lap and recur in the different members in the same way as the 

 identical species were shown to do, but nevertheless they pre- 

 sent a strong case. For example, 27 out of 46 cryptogams, 4 

 out of 7 cycads, and 11 out of 28 conifers have allies in the 

 Jurassic ; that is, over half of the species, exclusive of Dico- 

 tyledons, that admit of any comparison at all, show affinities 

 with Jurassic and Rhetic forms. And even this does not show 

 the full force of the evidence of this class, since there are many 

 genera, both of ferns and conifers, which predominate in the 

 Oolite and in the Rhetic, and are either wholly absent or very 

 rare in the Cretaceous, which yet have numerous representa- 

 tives in the Potomac, and therefore all the species of these 

 genera, whether they have closely related species in the Poto- 

 mac or not, bear directly upon this point. 



Thus Cladophlebis is represented by 22 species, only 14 of 

 which are related to forms found elsewhere. The genus is 

 chiefly Jurassic. 



Thinnfeldia is a large Jurassic genus ranging from the mid- 

 dle Trias, but found also in the Cretaceous of Spitsbergen 

 and Greenland. The Potomac species are, however, Rhetic in 

 type. 



Angiopteridium has no Cretaceous forms, and although there 

 are two alleged Miocene species, the nine Potomac forms are 

 strong lower Jurassic and Rhetic types. 



Thyrsopteris, leaving Stur's Culm species out of the account, 

 ranges from the Permian to the Malm of Portugal, held .to be 

 brackish-water Jurassic, called Wealden by Heer, but the genus 

 is chiefly Oolitic. It is the most abundant fern in the Poto- 

 mac formation, numbering forty species, only eight of which 

 admit of specific comparison with forms previously known. 

 Yet all of these forty species speak for the Jurassic age of the 

 deposits containing them. 



