FLORA OF OLDER POTOMAC FORMATION. 355 



with what he afterwards called the Fredericksburg beds, and the latter 

 with the James River beds. To the whole he gave the name of the 

 "Border belt." He recognized an "upper series," which inclifded not 

 only what he afterwards called the "Brooke" beds, but also most of the 

 formation as it exists in Mar^dand. As this has an important bearing 

 upon the Potomac of Maryland as now understood, it will be well to 

 bring it especially to the reader's attention: 



The lower series passes up into a higher system of beds, constituting the upper 

 series, which is marked by a smaller proportion of the white incoherent beds, so 

 characteristic of the lower, and by a predominance of clays of reddish, yellowish, 

 and bluish colors, and of reddish and yellowish sands. These clays and sands 

 increase in amount as we fohow the belt northward. Near Alexandria, between 

 Washington and Baltimore, and near the latter city, they constitute the whole of 

 the upper series. The material of these beds comes from the decay of the Azoic 

 on the west. These clays and sands also are very irregularly bedded. The sands 

 especially, are much affected by cross bedding. From Alexandria northward the 

 lower series is rarely seen, being too deeply buried. At Baltimore it appears in 

 the lowest white clays and sands dug in the base of the hills (see pp. 1.54-155; 

 Reprint, p. 42.) 



This paper contains the first mention of the "archaic dicotyledons" 

 of the Potomac formation, the discovery of which has thrown so great 

 light upon the origin of that subclass of plants and has caused such an 

 extensive readjustment in the geological classification of plants. On 

 this subject he says: 



With the plants above named, I find certain netted veined leaves, which by 

 their nervation can not be distinguished from Angiosperms. Had they been found 

 with Cretaceous or Tertiary plants I think no one would hesitate to consider them 

 as such. As; however, they occur with a well-marked upper Jurassic flora, I 

 hesitate to pronounce them to be Angiospermous plants without a more careful 

 study and extended comparison than I have as yet been able to make. They 

 are certainly not " Dictyophyllum " which is the genus of fossil ferns that stands 

 nearest to them. But when we find such a development of undoubted Angiosperms 

 in the lowest Cretaceous beds of New Jersey and of the west, we should expect to 

 find at least their ancestors in the Jurassic flora (see p. 156; Reprint, p. 44.) 



In speaking further of the Maryland beds he says: 



Great quantities of lignite occur in the clays of the upper series, especially the 

 clays which Mr. Tyson calls "u-on-ore clays," which are found between Washington 

 and Baltimore. In these clays, stumps of Cycads, belonging to at least two new 

 species of the genus Cycadoidea, are found. The blue clays at Baltimore yield some 



