FLORA OF OLDER POTOMAC FORMATION. 385 



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

 pahannock freestone, much more distinct plant impressions were found 

 than had ever been seen elsewhere under the same conditions, and a 

 considerable collection was made on July 27. 



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

 however, were not always favorable, and some of the views were not clear. 

 A few of them are fairly representative and may be introduced here 

 (see Pis. LXXVII, LXXVIII). 



Two other excursions were macie on August 5 and 8 to Gunstons Cove, 

 Masons Neck, and White House Bluff, for the purpose 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. Under the microscope I found a very 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 be 

 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. There 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 specimen that gave the same results to optical tests that the glauconite of 

 the greensand did. 



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

 Black Hills, but was resumed in October, and many other exposures were 

 examined. It was continued on pleasant days during the open winter of 

 1893-94. On January 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 taking 

 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 XLVUI — 05 25 



