PLANT LOCALITIES. 27 



Between Washington and P>altiiiiore lignite is the most important sub- 

 stance of organic origin found in it. The auiount of tliis seems to bo large, 

 and it occurs, as in Virginia, iu the form of logs changed to lignite in places 

 where the trees were entonil)ed. The characteristic mode of occurrence in 

 such cases is the appearance of the material in the form of entire trunks, 

 which have lain undisturbed until the present time. This is very diiferent 

 from the condition in which we hud the lignite of the Variegated Clays. 

 The lignite logs, as in Virginia, are embedded in gray clay. They occur 

 sometimes scattered iu the sand, but each log has its patch of clay sur- 

 rounding it. The difference between the mode of occurrence of the lignite 

 of the lower Potomac and that of the Variegated Clays is well shown 

 near Hanover, a station on the Baltimore and Ohio Railroad about fifteen 

 miles south of Baltimore. 



Near Hanover a thick mass of the Variegated Clay covers the lower 

 Potomac. It foruis the hills around the station, while the latter is barely 

 exposed in the beds of tlie largest creeks. Carbonate of iron here, and in 

 many other places, has for a long time been undergoing concentration in 

 the Variegated Clay, owing to concretionary processes. The result has 

 been tlio formation of nodules and crusts of ii'on, which for nearly one hun- 

 dred years have been obtained l)y open cuts. Around Hanover the.se pits 

 are numerous and extensive, disclosing well the character ami structure of 

 the clav. None of them seem to have penetrated through the clay to the 

 lower Potomac sand below. This clay has all the irregular features of dis- 

 turbed clay, that is, the material looks as if it w'ere nuide up of the con- 

 fusedly deposited debris formed by the erosion of older clay beds. The 

 lignite, which is abundant in some places, occurs as fragments, large and 

 small, mixed with the clay. No entire logs remaining in the position iu 

 which the trees were originally entombed have been seen. The material 

 w^as probably lignite and not wood when it was buried in the clay. 



Near the station there is a considerable creek, which has cut its chan- 

 nel below the base of the Variegated Clays. In one place the stream has 

 just cut down to the top of the lower Potomac sand, so as to expose a few 

 feet of it. The Variegated Cla}' is seen to rest on the eroded top of this. 

 A number of isolated logs of lignite are found here also, much flattened by 



