26 THE POTOMAC OE YOUNGER MESOZOTC FLORA. 



evidence goes, the indications are that tlie variegated clay group is of the 

 same age as the plastic clays of New Jersey. 



While showing considerable variations in mineral character and phys- 

 ical properties, this group, when unweathered, is mainly composed of an 

 ash-gray to dark lead-gray plastic cla}^, often mixed with more or less sand. 

 Quite commonly much of it is a pure highly plastic clay. In the upper, 

 and especially the exposed portions, the iron diffused in the clay has become 

 irregularly oxidized, so that spots, patches, streaks, etc , of red and yellow 

 colors are mingled with the normal gray. From this feature the names 

 "Variegated Clays" and "Mottled Clays'' have been taken. 



This group during a very long period has been worked locally for the 

 iron present in it. The clay is without bedding or regular structure of 

 any kind. It is much affected by slickensides, and, in short, has many of 

 the features seen in the disturbed cla^^s of the lower Potomac formation. 

 It has a great deal of disseminated lignite, mostly in the form of small 

 fragments. Sometimes the lignite is intimately mixed with the clay, and 

 if the particles are very small the clay is made black by them. The 

 lignite, unlike most of that of the lower Potomac, does not seem to have 

 been formed from the alteration of trees buried during the accumulation 

 of the clay, but appears to have been already in the form of lignite when 

 introduced into the clay. When we consider that the amount of lignite 

 in the lower Potomac is very large, and note tluxt the Variegated Clay 

 rests on it and is certainly partly at least made up of the ruins of that 

 formation, the idea is strongly suggested that the comminuted lignite of 

 these clays comes from the fragments of the lignite logs so common in 

 the lower Potomac 



What has been said above belongs more properly to the geological 

 portion of this memoir, but it is necessary to anticipate to this extent in 

 order to render intelligible what will be said about the probable horizon of 

 the Baltimore and other Maryland plants, whose exact geologic position has 

 been left doubtful by their discoverers. Owing to the depth at whicli the 

 lower Potomac of Maryland usually occurs, the exposures, so far as found, 

 are few and limited in extent. Detailed search has not. been made in it for 

 plant-impressions and but few have been found. 



