INTEODUCTION. 7 



lie was kind euough to promise to sliow me the spot. Under his guidance 

 I visited this phmt locality and found the exposure in the form of a small 

 ledge of rock, showing an outcrop only a few feet in length. Here I 

 made my first collection from the Potomac strata. Among the specimens 

 found were some that appeared to be certainly dicotyledons. These plants, 

 and many others new and of great interest found tljere, induced me to 

 make thorough search of the spot. The experience gained at Fredericks- 

 burg taught me how to look for fossils in this formation. In the course of 

 several years, during which examinations were made, the number of plant 

 localities found and the amount of material obtained proved to be unex- 

 pectedly great. 



The mode of occurrence of the plants in the Potomac formation de- 

 serves a brief general description. There is nowhere any continuous 

 stratum affording plant- impressions. Wlien found at any locality and on a 

 given horizon it rarely happens that even in the innnediate vicinit}' any 

 trace of the plant-bearing bed can be found. One can depend upon nothing 

 except what is seen on the spot to contain plants. Even this material as a 

 rule, wlien followed in working, soon gives out, often "suddenly and un- 

 expectedly. In the sandy portions of the formation we find little besides 

 silicified wood, and even this is very rare. 



Lignite is abundant, but it always occurs enveloped in clay. Tliis, 

 when damp, is, with hardly an exception, dark gray to black. All the 

 determinable portions of plants occur in clay, which, as a rule, is but little 

 indurated. It is usually (piite plastic and tenacious. In rare cases the 

 material is somewhat indurated, so as to approach in nature a shale. Two 

 colors are common in the clay; one is pale reddish brown or chocolate; 

 the other wlien damp is dark lead-gray to nearly black. The first is more 

 commoidy the cai-rier of identifiable plants, but even this clay is by no 

 means usually fossiliferous. In many cases it is without a trace of vege- 

 table matter, and in others it contains nothing but small diffused bits of 

 plants. The gray clay sometimes contains good impressions, but more 

 commonly only lignite. 



Both kinds of clay occur in two forms: (1) They are tbund undisturbed 

 and as originally deposited; (2) they appear disturbed, that is, torn up from 



