FLORA OF OLDKK POTOMAC FOK.MATION. 3(55 



■e 



From tlic lowest layer 1 have taken oul |)laiils only of a low type of stnietur 

 resembling alga' and nitellas; Irom the next layer above, e(iiiiseta' and- ferns with 

 strange vhie-like stnicture: from the layer a few feet higher, buds and twigs of 

 trees allied to the cvpress an<l icdwoods of California, as also leaves of ferns iiaving 

 the form of those of the Gingko; from the fourth layer other ferns, coniferous stems, 

 buds, and scales, with some leaves of dicotyledons resemlding sassafras: and from 

 the ujjper layer leaves whicii resendjle those of the hawthorn, magnolia, willow, and 

 hendock. 



The less distinctly stratified elay overlying tliese is rich in lignite, often con- 

 taining the trunks and lind)s of nearly entire trees, some of which have been found 

 with s])nice-like cones and needle-sha])ed leaves. 



The contimiation of this l)ed tipwards is composed of the iron ore chiys wliicli 

 form such conspicuous hills and ridges along tlie road leading to Washington. In 

 this member of the series lie the extensive layers of carbonate of iron, the richest 

 of which occur near the base, while the nodules ami oxidized lumps are found nearer 

 the surface. Tiie extension of this bed still higher, at various levels, displays the 

 red and white variegated clays, such as we se(> in large areas in crossing the country 

 south and east of the iron ore hills (see pp. 48, 49). 



It was remarked that the eycadean trtinks collected hy Txsoii in 

 Maryland were deposited in the museum of the Maryland Academy of 

 Sciences. It was there that our party saw them in 1885 and had photo- 

 graphs of them made, to illustrate Professor Fontaine's monograph of 

 the Potomac flora. After the organization of the museum of the Johns 

 Hopkins University the Maryland Academy, being obliged to contract 

 its quarters, donated its paleontological collections to the university-. 

 The cycads were transferred among the rest, and are still there. Prof. 

 Wm. B. Clark gave an account of the transfer in 1888," with appropriate 

 acknowledgments. 



The first of Mr. McCiee's papers d(\scribing "Three Formations of the 

 Middle Atlantic Slope," published early in 1888,'" is chiefly devoted to the 

 Potomac formation. From it we learn that he had extended his investi- 

 gations much farther to the northeast than had l^een reached by the 

 ' expedition of 1885, and had studied the contact of the coastal plain 

 with the underlying older rocks through Delaware and Pennsylvania into 

 New Jersey. He referred the "Bryn Mawr gravel" (p. 130), the "feri-u- 



".Joliiis Ilopkin.s University Circulars, Vol. VII, No. 6.5, April, 1888, p. 67. 



f-Tliroe Formatiori.s of tli<^ Middle .Vtlanlic Slope, l)y W J McGee: Am. Jour. Sei., 3d ser., Vol. XXXV, 

 February, 1888, pp. 120-1 tJ: April. 1S,S.S, pp. 328-388; May, 1888, pp. 3()r-3SS: .haic, 1S8S, pp. 4-IS-466, 

 pis. ii and vi. 



