FLORA OF OLDER POTO^LIC FORMATION. 385 



Cockpit Point, in rocks overlying; these clays and representing; the Rap- 

 pahannock freestone, much more distinct plant impressions were found 

 tlian had evei- Keen s(>(mi el.sewhere under the same conditions, and a 

 considerable collection was made on .Inly 27. 



Th(> kodak was in use during this entire expedition. The conditions, 

 how(n-ei', were not always favorable, and .some of the views were not clear. 

 A few of them are fairl>' representative and may Ije introdiicetl here 

 (see Pis. LXXVII, LXXVIII). 



Two other excursions were maae on August 5 and S to Gunstons Cove, 

 Masons Xeck, and White House Bluff, for the piu'pose of reexamining 

 these exposures in the light of information gained since they were last seen, 

 especially relative to the green basal clay which had so greatly puzzled me 

 on previous occasions. It proved to be the same as at Cockpit Point. 

 Specimens of the latter had been sent to Prof. J. S. Diller for analysis, and 

 under date of November 28, 1893, he reported upon them as follows: 



To-day I examined carefully the greenish sand you gave me from "Cockpit 

 Point, green basal clay from under plant bed, July 27, 189.3." When put in water 

 the sand becomes more clearly green. Lnder the microscope I founil a verj' con- 

 siderable amount of green mineral fragments. Some of these were clearly pleochroic 

 and doubly refracting, like hornblende and chlorite. They were evidently not 

 glauconite, which is not pleochroic and gives aggregate polarization and could l)e 

 quite easily distinguished from such well-marked mineral fragments. 



There are plenty of these green particles of hornblende and chlorite, and it 

 is to these that the specimen owes its color, chiefly if not wholly. Tlu^re are a few 

 grains which, on account of their opacity, could not be clearly determined, and 

 it is possible that they are glauconite, but of this I could get no proof. 



I compared the above material with typical greensand of New Jersey, in which 

 the glauconite is abundant and characteristic, but I could not find any grains in 

 your s{)ecimen that gave the same results to optical tests that the glauconite of 

 the greensand tlid. 



The work on the Potomac was interrupted by an expedition to the 

 Black Hills, but was resumed in October, and many other exposm-es were 

 examined. It was continued on pleasant days during the open winterof 

 1893-94. On Januarys 14, 1894, an excursion was made up Back Lick 

 Run southwest of Alexandria, where some excellent exposures occur. Mr. 

 Victor Mason accompanied me on this excursion and assisted me in J-aking 

 a number of kodak views. The relations of the beds are nearly the same 

 here as on Sixteenth street in Washington, except that the green chloritic 



MON xi-viii — 0.5 2.5 



