30 THE POTOMAC OR YOUNGER MESOZOIC FLORA, 



A oreat <leal of excavation has been carried on in and around Balti- 

 more, and by this means the character of the formations there has been 

 well disclosed. Lignite is abundant, showing two very distinct modes of 

 occurrence. At all horizons in the Variegated Clays, except at what has 

 been generally supposed to be its base, this material occurs, as at Hanover 

 station, in fragments mixed irregularly with the clay. But at several 

 places near the supposed base of the Variegated Clays it has been found in 

 entire lofs, either isolated or aggregated in masses. The material seems to 

 have been undisturbed since the change of the trees to lignite. I have not 

 seen these lignites, and hence can not undertake to determine positively the 

 formation in which they are found; but from what I have seen of the mode 

 of occurrence of the lignite in the lower Potomac and in the Variegated 

 Clays, 1 feel sure that the undisturbed logs occur only in the former. 



After this discussion, which is largely an anticipation of matter more 

 proper for the geologic description of the formation, but which is necessary 

 on account of the doubt attending the true horizon of the first discovered 

 Maryland plants, we may notice the occurrence of Tyson's cycad trunks. 



Mr. Tyson has fortui:ately left in his unpublished notes some record 

 of the finding of his cycad trunks. By Mr. Tyson and by all others they 

 have been attributed to the Variegated Clays, but this reference is probably 

 not correct. He says that on May 5, 1859, lie went to Latcliford's ore 

 mine (near Contee's) and got fragments of Saurian bones just exhumed. 

 They occurred, according to him, near a lignite tree about sixty feet long. 

 From what was said above this would seem to show that the Saurian bones 

 were found in the lower Potomac clay and not in the Variegated Clay. 

 He also states that he went to Dr. Jenkins's place and noted the position 

 of the cycad trunks to be at or near the base of the lead-colored clays. 

 This statement is of importance, for the portion called by him " lead-col- 

 ored clays" forms the base of the Variegated Clays. This position makes 

 it quite probable that the horizon of the trunks is reall}^ the upper part of 

 the lower Potomac. 



It should be noted that the trunks are thoroughly silicified, and it is 

 possible that they may have been silicified in the lower Potomac sand, and 

 then on the erosion of this, during the deposition of the clay containing 



