8 THE POTOMAC OE YOUNGER MESOZOIC FLORA. 



the original bed and redeposited in an irregular manner. The undisturbed 

 clay often possesses a somewhat regular structure. It is then either lami- 

 nated or cleavable, especially if plants are present. This form of the clays 

 gives almost all of the well-preserved specimens. These undisturbed clays 

 occur in more or less extended lenticular sheets that finally thin out and 

 disappear, showing in their course many irregularities. Often they are 

 found in pockets of limited extent. 



The clays are commonly found disturbed. They then appear in the 

 form of masses or small particles that have been torn up from some pre- 

 existing clay layer, transported to a greater or less distance, and deposited 

 either singly enveloped in the sands, or together in irregular pockets and 

 layers mixed with some sand. If abundant, the redeposited clay particles, 

 when cemented together by pressure, form la3'ers, at first sight not unlike 

 the undisturbed clay beds. It is easy to detect these deposits on attempt 

 ing to work out plant impressions occurring in them. They lack the lami- 

 nated structure and cleavage of the undisturbed material. The clay is 

 much mixed with sand, and is confusedly heaped together, contorted, and 

 affected by slickensides. The contained fossils are very fragmentary, and 

 lie at any angle in the material, so that it is rarely possible to extricate them 

 without destroying the specimens. 



In some cases the redeposited masses are large enough to retain the 

 original comparatively regular structure, especially if they have not been 

 transported far. In such cases good impressions may be obtained. In 

 some cases a remnant of the original bed may be found covered by the dis- 

 turbed clay. There is no essential difference between the plants in the 

 original beds and those in the redeposited clays, and hence no very long 

 period of time had elapsed between the deposition of the original bed and 

 that of the disturbed material. These confused and disturbed clays may 

 occur at any horizon in the Potomac, and they are common at and near its 

 top. They are often mixed with coarse sand, and they even contain peb- 

 bles, sometimes of large size. It is not unusual to find one of these dis- 

 turbed clays resting on a cobblestone layer. 



A common mode of occurrence for the clay of both colors is in the 

 form of isolated masses and particles imbedded in the sand. In some cases 



