Fr/>H.\ or ()ij)i:h poTo^[A(• FoinrvTioN'. :)55 



willi what li(> aftcrwai'ds cnllcil the I'Vcdcrickshiii-g IkhIs, and the lattci- 

 with the .hiiiics IJivei' beds. 'I'o tlic whole he ffavo the nanie of tlic 

 ■■Border holt." He i-cM-ogiiized an "'uppei' series," which inchided not, 

 only what he aftei-wards ealled the "Brooke" l)(>(ls. hut also most of th(> 

 t'oi-niation as it exists in Maryland. As this has an iinpoi'tant heariuii 

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

 hi'iiiii' it (>sp(>cially to the r(>ad(>i''s attention: 



Till' lower scrit-s |);iss('s u|) iiilci a liii^dicr sysiciii of Ix'ds. cotisiituting the u|)|)('r 

 scrios, w'hicl) is iiiiU'kcd by a small<'r proportion of llic white iiicolicront \>ods. so 

 characteristic of the lower, and l)y a predominance ol' clays of reddish, yellowish, 

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

 increase in amount as we follow the belt noi-thwai'd. Near Alexandria, between 

 Washini;ton and Baltimore, and near the latter city, they constitute the whole of 

 the u[)per series. The material of these l)eds comes from tlH> d('ca\' of the Azoic 

 on the west-. These clays and sands also are \erv irrei,nilarly bediled. The sands 

 esj)eeially, are much aflfected by cross betltlin<;. From Alexantlria northward the 

 lower series is rarely seen, being too deejily buried. At Baltimore it apjx'ars in 

 th(> lowest white clays and sands dug in the base of the hills (see p]). l.")4 1.")."): 

 Iteprint, 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 plant.s. On 

 this subject he says: 



AVitli the plants above named, I find certain netted veined leaves, which bv 

 their nervation can not be distinguished from Angiospernis. Had the}' 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 up))er Jurassic flora, I 

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

 study and extended comparison than 1 have as yet been able to make. They 

 are certainly not " I)ict\'opliyllum" which is the genus of fossil ferns that stands 

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

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

 lind at least their ancestors hi the Jurassic flora (sec p. I. "id: Reprint, p. 44.) 



In speaking fiu'thei- of the Maryland beds he says: 



Great ciuantities of lignite occur in the clays of the upjier series, es|)ccially the 

 clays which Mr. TA'son calls "iron-ore clays," which are found between Washington 

 and Baltimore. In these clays, stumps of Cj-cads. belonging to at least two new 

 S])ecies of the genus Cycadoide;i. are found. The blue clays at Baltimore yield some 



